Preheat oven to 375°F (190°C)
Cook macaroni in salted water until just al dente
Blanch cauliflower florets until tender; drain
Melt butter in a saucepan
Whisk in flour and cook briefly
Add mustard powder
Slowly whisk in milk to form a smooth sauce
Stir in cheeses until melted
Season with salt, pepper, and cayenne
Combine pasta, cauliflower, and sauce
Transfer to baking dish
Mix breadcrumbs with thyme
Sprinkle over mac and cheese
Bake until golden and bubbling