This bubbly and cheesy dish makes for a delicious dinner party side. Serve with Salt and Pepper Crusted Rack of Lamb and Wilted Mustard Greens.
Preheat oven to 350 degrees. Butter a shallow 2-quart baking dish and set aside.
Bring a large pot of salted water to a boil. Add cauliflower and cook until slightly tender, 2 to 3 minutes. Drain and rinse under cold water; transfer to a large bowl and set aside.
Melt butter in a small saucepan over medium-low heat. Add leeks, shallots, red-pepper flakes, and thyme; cook, stirring, until softened, about 5 minutes. Stir in flour until well combined. Slowly add milk, stirring constantly, until well combined. Increase heat to medium-high and bring to a boil; let boil 2 to 3 minutes until thickened. Remove from heat and stir in 3/4 cups cheese; season with salt and pepper.
Pour leek mixture over cauliflower mixture and stir until cauliflower is well coated; transfer to prepared baking dish. Sprinkle remaining 3/4 cup cheese over top. Transfer to oven and bake until bubbly, about 25 minutes.
Meanwhile, preheat broiler. Transfer baking dish to broiler; broil until golden brown, 5 to 8 minutes. Let cool 5 to 10 minutes before serving.