1. Coat a 21/2-quart baking dish with cooking spray. Preheat the oven to 400 degrees . In a large pot, cook pasta for 1 minute less than package instructions call for, adding cauliflower to pot 4 minutes before pasta is done. Drain thoroughly and return pasta and cauliflower to pot.
2. Meanwhile, whisk together 1/2 cup milk, flour, mustard, thyme, 1/2 teaspoon salt, garlic powder and pepper in a small bowl. Heat remaining milk in a medium saucepan over medium heat until steaming but not simmering. Whisk flour mixture into milk and continue cooking, stirring often, until thickened and bubbling, about 4 minutes. Remove from heat and stir in Gruyere and Parmesan until melted and smooth.
3. Stir cheese sauce into pasta and cauliflower and transfer to prepared baking dish. Combine bread crumbs, oil, paprika and pinch of salt in a small bowl. Top macaroni and cheese with bread crumb mixture and bake until bubbling and golden, 22 to 25 minutes. Let sit 10 minutes before serving.