Preheat the oven to 400 degrees F. Butter a 9-by-13-inch baking dish. Fill a large bowl with ice water.
Bring a large pot of generously salted water to a boil. Cook 3 1/2 cups of the cauliflower until crisp-tender, about 2 minutes. Remove the florets with a slotted spoon, and plunge in the ice water to stop the cooking. When cool, strain and reserve.
Return the water in the pot to a boil, and cook the pasta according to the package directions for al dente. Strain the pasta, shaking the colander a few times to drain excess water.
While the pasta is cooking, heat 4 tablespoons of the butter over medium heat in a large pot. When the butter foams, add the leeks, mushrooms and 1/4 teaspoon salt, and cook, stirring occasionally, until the leeks are lightly browned and the mushrooms are tender, about 2 minutes. Sprinkle the flour over the vegetables, and cook, stirring, for 30 seconds. Whisk in the half-and-half and bay leaf, and cook, continuing to whisk, until the mixture comes to a boil and thickens. Lower the heat to a simmer, add 2 teaspoons salt and simmer 1 minute. Remove the pot from the heat, take out the bay leaf and let sit for a minute. Add 3 1/2 cups of the Cheddar, and stir until smooth. Gently fold the cooked cauliflower and pasta into the cheese sauce, and transfer the mixture to the baking dish.
Pulse the remaining 1/2 cup cauliflower in a food processor until it resembles couscous. Melt the remaining 4 tablespoons butter in the microwave, about 1 minute. Add the melted butter, breadcrumbs, thyme and 1/2 teaspoon salt to the food processor and pulse until combined. Sprinkle evenly on top of the pasta mixture. Sprinkle on the remaining 1/2 cup Cheddar, and bake until bubbly and brown on top, 25 minutes. Let stand 5 minutes. Serve hot.