Brussels sprouts gratin with a crispy, cheesy topping!
Heat the oven to 425°F. Put the Brussels sprouts in a 9x13-inch baking dish, and toss with 2 tablespoons of the melted butter, 3/4 teaspoon salt, and 1/4 teaspoon pepper. Spread the sprouts evenly in the dish and roast, tossing once halfway through, until tender and browned in spots, 25-30 minutes.
While the sprouts roast, place the bread crumbs in a medium bowl. Pour the remaining tablespoon of melted butter over the crumbs and sprinkle with 1/8 teaspoon salt. Mix in the Gruyère. Set aside.
When the sprouts are tender and browned, pour the cream evenly over the sprouts and sprinkle with 1/4 teaspoon salt. Continue baking until the cream has thickened to a saucy consistency, about 10 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 bread crumb mixture. Broil the gratin until the crust is golden brown, 3-5 minutes (keep a close eye on it so it doesn't burn).
Make ahead tip: This recipe can be prepared a day ahead of time. Reheat in a 350°F oven, uncovered, until warm throughout and crisp on top, 10-15 minutes.