- STEP 1
Boil Brussels sprouts in a large pot of boiling salted water until crisp-tender, 5 to 8 minutes. Drain and run under cold water to cool; spread in a single layer, on a paper towel-lined baking sheet; dry completely.
- STEP 2
Cook garlic in butter in a large saucepan over medium heat until fragrant, 1 minute. Sprinkle with flour; cook, whisking, 1 minute. Slowly whisk in half-and-half. Simmer, whisking often, until thickened, 6 to 7 minutes. Remove from heat and whisk in 6 ounces (about 1 1/4 cups) Gruyère; fold in Brussels sprouts. Season with salt and pepper.
- STEP 3
Transfer to a buttered 3-quart baking dish. Top with remaining Gruyère (about 3/4 cup). Bake at 375 degrees F until bubbly and top is golden brown, 25 to 30 minutes. Let stand 5 minutes before serving.