Baked Penne with Cauliflower and Cheese

4 cups 1-1/2-inch cauliflower florets (about 1 lb.; from 1/2 head) 1 medium onion, sliced 2 cloves garlic, peeled Fine sea salt or table salt 12 oz. dried penne 2 cups 1% milk 1 tsp. dry mustard Freshly ground black pepper 2 tsp. chopped fresh thyme 2 oz. coarsely grated sharp white Cheddar (about 1/2 cup) 1-1/2 oz. finely grated Parmigiano-Reggiano (1-1/2 cups using a rasp grater) ________________________________________

Position a rack in the center of the oven and heat the oven to 375°F. Put the cauliflower, onion, and garlic in a steamer basket set over 1 inch of boiling water in a 6- to 8-quart pot. Cover and steam until the cauliflower is tender, 10 to 15 minutes. Transfer the cauliflower, onion, and garlic to a blender. Fill the pot three-quarters full of salted water and bring to a boil over high heat. Add the pasta and cook for 3 minutes less than the package timing. Drain and return the pasta to the pot. While the pasta cooks, add 1 cup of the milk, the dry mustard, 1 tsp. salt, and 1/2 tsp. pepper to the vegetables in the blender and purée until smooth. Transfer to a 3-quart saucepan and stir in the remaining 1 cup of milk and the thyme. Heat over medium-low heat until hot but not boiling, about 3 minutes. In a small bowl, mix the Cheddar and Parmigiano. Add all but 1/2 cup of the cheese to the sauce and stir until the cheese is melted. Add the sauce to the pasta and stir to combine. Transfer the pasta and sauce to an 8-inch square baking dish and sprinkle with the remaining cheese. Bake until heated through and the cheese is beginning to brown, 20 to 30 minutes by Ellie Krieger