2 lbs Brussels sprouts, stem ends trimmed and outer leaves removed; sprouts cut in half through the stem end
3 Tbsp unsalted butter, melted
Kosher salt and freshly ground black pepper
1 cup coarse fresh breadcrumbs (from a baguette or other white bread)
3/4 oz (1/4 cup) finely grated Gruyere cheese
1 1/4 cups heavy cream (whipping cream), room temperature
1) Heat the oven to 425F. Put Brussels sprouts in a shallow baking dish that will hold them in a snug single layer (a 9x13 rectangle or slightly smaller oval is good). Toss with 2 Tbsp of the melted butter, 3/4 tsp salt, and several grinds of pepper. Spread them evenly in the dish and roast, tossing once or twice, until browned in spots and tender when pierced with a knife, 25-30 minutes.
2) While the sprouts roast, combine the breadcrumbs with the remaining 1 Tbsp melted butter, and 1/8 tsp salt in a small bowl. Mix in Gruyere cheese.
3) When the Brussels sprouts are tender and browned, pour the cream over them and continue baking until the cream has thickened to a saucy consistency and coats the sprouts, about 5-7 minutes. Remove the pan from the oven. Set the oven to broil and position a rack to 6 inches below the broiler. Sprinkle on the breadcrumb mixture. Broil the gratin until the crust is deep golden brown, about 5 minutes.