Cauliflower Gratin with Leeks and Gruyere

FOR THE TOPPING: Pulse bread, butter, salt, and pepper in food processor until mixture resembles coarse crumbs, about ten 1-second pulses; set aside.

FOR THE FILLING: Adjust oven rack to middle position; heat oven to 450 degrees. Bring 4 quarts water to boil in Dutch oven or stockpot over high heat. Add 1 tablespoon salt and cauliflower; cook until outsides are tender but insides are still slightly crunchy, 3 to 4 minutes. Drain cauliflower in colander and rinse under cold running tap water until no longer hot. Leave cauliflower in colander to drain.

Heat butter in large skillet over medium heat; when foam subsides, add shallot and leeks and cook until softened, about 4 minutes. Add garlic and cook until fragrant, about 30 seconds; stir in flour until combined, about 1 minute. Whisk in cream and bring to boil. Stir in nutmeg, cayenne, 1/4 teaspoon salt, pepper, thyme, and 1/2 cup cheese until incorporated. Off heat, gently stir in cauliflower until evenly combined. Transfer mixture to 11- by 7-inch (2-quart) gratin dish. Sprinkle remaining 2 tablespoons cheese evenly over surface, then sprinkle evenly with bread crumb topping. Bake until golden brown and sauce is bubbling around edges, 10 to 12 minutes. Serve immediately.