This Brussels sprouts gratin has been a holiday and weeknight favorite in my kitchen for years. I first adapted it during a chilly November when I wanted a vegetable side that felt indulgent without being heavy, and roasting the sprouts before baking them in a white sauce became the turning point. The result is a dish with edges that crisp and caramelize while the interior stays tender, all wrapped in a silky, thyme-scented sauce with a golden Gruyère crust. Family members who normally avoid Brussels sprouts now ask for seconds, and the leftovers—if any—reheat beautifully.

What makes this gratin special is the balance between the vegetal snap of the sprouts and the nutty, savory melt of the cheeses. Roasting concentrates the sprouts' natural sugars and gives you those toasty notes; the roux-thickened sauce brings a creamy mouthfeel without requiring heavy cream, so the overall finish feels rich but not cloying. I often make this when entertaining because it travels well from stovetop to oven, can be prepared ahead, and pairs with a broad range of mains from roasted chicken to baked salmon.

Ingredients

• Brussels sprouts (16 oz): Choose firm, bright green heads with tight leaves. Trim the tough stem ends and remove any damaged outer leaves, then halve them lengthwise to maximize caramelization. Small-to-medium sprouts roast most evenly.
• Kosher salt and black pepper: Salt the sprouts twice—lightly before roasting and again in the sauce if needed—to layer seasoning. Adjust pepper to taste for a gentle bite.
• Olive oil spray: A light coating helps the sprouts brown without excess oil. If you don't have a spray, use about 1 tablespoon olive oil tossed with the sprouts.
• Butter (1/2 tbsp): Adds richness to the shallots and roux. Use unsalted to control sodium, or salted if that's what you have on hand.
• Shallots (1/3 cup, chopped): Mild and slightly sweet, they melt into the sauce and provide a subtle onion base. Red or white onion will work in a pinch but shallots are preferred.
• All-purpose flour (2 tsp): Forms the roux to thicken the sauce. For a gluten-free version substitute a 1:1 gluten-free flour blend.
• Fat-free milk (3/4 cup): Keeps the sauce light while still allowing the cheese to melt smoothly; you can use 2% milk for slightly more creaminess.
• Fresh thyme (1 tsp): Adds an herbal, slightly lemony lift that cuts through the richness; strip leaves from the stem and add toward the end of cooking the sauce.
• Parmesan (1 tbsp, grated): Brings salty, umami depth—look for Parmigiano-Reggiano for best flavor.
• Gruyère (2 oz, grated, divided): Nutty and melty, it creates that signature gratin crust. Reserve half for mixing into the sauce and half for topping.

Instructions

Preheat and prepare: Preheat the oven to 400°F. Spray an 8"x12" gratin dish or shallow casserole with olive oil spray. Halve 16 oz of trimmed Brussels sprouts and place them cut-side up in the dish. Season lightly with 1/4 teaspoon kosher salt and freshly ground black pepper to taste. The initial seasoning helps the natural sugars caramelize while roasting. Roast the sprouts: Spray a light coating of olive oil over the sprouts (or toss with 1 tablespoon olive oil). Place the dish in the lower third of the preheated oven and roast for 15 minutes. Remove, give the sprouts a gentle toss to encourage even browning, then return to the oven for another 10 minutes. You want golden-brown edges and tender centers—pierce a halved sprout with a knife to check for doneness. Sauté the shallots: While the sprouts roast, heat a medium nonstick pan over medium heat. Add 1/2 tablespoon butter and let it melt, then add 1/3 cup chopped shallots. Cook for 4 to 5 minutes until the shallots are soft and fragrant but not browned—softening keeps the sauce smooth and avoids bitter bits. Make the roux and white sauce: Sprinkle 2 teaspoons flour over the softened shallots and whisk or stir constantly for 1 to 2 minutes to cook off the raw flour taste. Slowly add 3/4 cup fat-free milk while stirring or whisking to break up any lumps. Reduce to medium-low and cook for about 4 minutes, stirring, until the sauce thickens and coats the back of a spoon. The cooked roux stabilizes the sauce so it holds the cheese without separating. Finish the sauce: Remove the sauce from the heat and stir in 1 teaspoon fresh thyme leaves, 1 tablespoon grated Parmesan, and 1 ounce (half) of the grated Gruyère. Stir until melted and glossy. Taste and adjust seasoning—add a pinch more salt if the cheese isn’t salty enough. Assemble and bake: Pour the cheese sauce evenly over the roasted sprouts in the gratin dish. Sprinkle the remaining 1 ounce Gruyère evenly over the top. Bake at 400°F for 15 minutes, or until the top is lightly browned and the sauce is bubbly. Let rest 5 minutes before serving to allow the sauce to set slightly.

Storage Tips

Cool the gratin to room temperature before refrigerating. Store in an airtight container or cover the dish tightly with plastic wrap; it will keep 3 days in the fridge. For freezing, assemble the gratin in a freezer-safe dish, cover tightly with foil, and freeze up to 3 months. To reheat from frozen, thaw overnight in the fridge, then bake uncovered at 375°F until bubbling and the top is golden, about 25–35 minutes. When reheating refrigerated leftovers, a 350°F oven for 10–15 minutes restores crispness better than the microwave.

Ingredient Substitutions

If you don't have Gruyère, use a good melting cheese such as Emmental, Comté, or mild aged cheddar—each will change the flavor slightly: cheddar adds sharper tang, Emmental keeps it buttery. For dairy-free diets, make a béchamel with a plant-based butter and unsweetened almond or oat milk and use dairy-free grated Parmesan substitute and a vegan melting cheese, though texture will be less traditional. Swap fat-free milk for 2% or whole milk for a silkier sauce. Gluten-free flour blends work 1:1 for the roux; alternatively, whisk in 1/2 teaspoon cornstarch dissolved in 1 tablespoon cold milk as a thickener.

Serving Suggestions

Serve this gratin alongside roasted poultry, pork tenderloin, or pan-seared salmon. For a full vegetarian meal, pair with a brown-butter sage gnocchi or a warm lentil salad. Garnish with extra thyme sprigs or a few crispy fried shallot rings for height. A squeeze of lemon on the side brightens the richness for palates that prefer an acidic counterpoint. For holiday spreads, place the gratin next to mashed potatoes and a simple green salad to balance the table.

Cultural Background

Gratins are rooted in French home cooking—traditionally a shallow baking dish topped with breadcrumbs or cheese and baked until crusty. This adaptation highlights Brussels sprouts, a vegetable with Flemish origins that became popular across Europe and the U.S. in the 20th century. Combining roasted brassicas with a béchamel-style sauce nods to classic French technique while embracing modern roasting methods to concentrate sweetness and texture.

Seasonal Adaptations

In winter, boost the gratin with a handful of halved chestnuts or tossed roasted pearl onions for additional body and seasonal character. In spring, swap thyme for lemon zest and tender parsley for a brighter finish. For autumn, add a small pinch of nutmeg to the sauce and finish with toasted hazelnuts for crunch. Adjust cheese choices with seasonal availability—an aged Gruyère in winter or a young, creamier variety in summer.

Meal Prep Tips

For streamlined meal prep, roast the trimmed and halved sprouts and refrigerate them in an airtight container for up to 24 hours. Make the sauce up to a day ahead and refrigerate separately; gently rewarm and whisk before combining with the sprouts. Assemble the gratin shortly before baking to avoid a watery top. If transporting to a potluck, bake until the top is just set but not deeply browned, then finish under a broiler at the host's house for a fresh golden crust.

Whether you're serving it for a holiday dinner or a cozy weeknight, this gratin is a reliable way to make Brussels sprouts feel celebratory. I hope it becomes a dish you return to whenever you want a vegetable course that feels like the centerpiece.