Preparation

• Heat the oven to 425°F. Put the Brussels sprouts in a shallow baking dish that will hold them in a snug single layer (a 9×13-inch rectangle or slightly smaller oval is good). Toss with 2 Tbs. 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 to 30 minutes.
• While the sprouts roast, combine the breadcrumbs with the remaining 1 Tbs. melted butter and 1/8  tsp. salt in a small bowl. Mix in the Gruyère.
• When the Brussels sprouts are tender and browned, pour the cream over them and conntinue baking until the cream has thickened to a saucy consistency and coats the sprouts, 5  to 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.