Pre-roasting the sprouts make them rich and nutty. Make a quick, creamy Mornay sauce, using a combination of Gruyere and Parmesan cheeses, to bind the gratin.
Add crunch, and top the gratin with toasted panko bread crumbs and more nutty Gruyere. If you can find only large Brussels sprouts (greater than 1 1/2 inches in diameter), quarter them.
Brussels Sprout Gratin:
Start to finish: 1 hr 15 minutes
2 1/2 pounds Brussels sprouts, trimmed and halved
1 tablespoon vegetable oil
Salt and pepper
3 tablespoons unsalted butter
1/4 cup panko bread crumbs
1 shallot, minced
1 garlic clove, minced
1 tablespoon all-purpose flour
1 1/4 cups heavy cream
3/4 cup chicken or vegetable broth
2 ounces Gruyere cheese, shredded (1/2 cup)
1 ounce Parmesan cheese, grated (1/2 cup)
Pinch ground nutmeg
Pinch cayenne pepper
Adjust oven rack to middle position and heat oven to 450 F. Grease 13 by 9 inch baking dish. Toss Brussels sprouts with oil, 1/2 teaspoon salt, and 1/4 teaspoon pepper in prepared dish. Bake until sprouts are well browned and tender, 30 to 35 minutes. Transfer to wire rack and set aside to cool for at least 5 minutes or up to 30 minutes.
Melt 1 tablespoon butter in medium saucepan over medium heat. Add panko and cook, stirring frequently, until golden brown, about 3 minutes. Transfer to bowl and stir in 1/4 teaspoon salt and 1/4 teaspoon pepper; set aside. Wipe saucepan clean with paper towels.
Melt remaining 2 tablespoons butter in now-empty saucepan over medium heat. Add shallot and garlic and cook until just softened, about 1 minute. Stir in flour and cook for 1 minute. Slowly whisk in cream and broth and bring to boil over medium-high heat. Once boiling, remove from heat and whisk in 1/4 cup Gruyere, Parmesan, nutmeg, cayenne, 1/4 teaspoon pepper, and 1/8 teaspoon salt until smooth.
Pour cream mixture over Brussels sprouts in dish and stir to combine. Sprinkle evenly with panko mixture and remaining 1/4 cup Gruyere. Bake until bubbling around edges and golden brown on top, 5 to 7 minutes. Let cool for 10 minutes before serving`.