Preheat oven to 350°. Place 1 in. of water in a Dutch oven; add cauliflower. Bring to a boil. Reduce heat; cover and cook until crisp-tender, 4-6 minutes. Drain and pat dry.
Meanwhile, in a large saucepan, saute onion in butter until tender. Stir in the flour, salt, mustard and pepper until blended; gradually add milk. Bring to a boil; cook and stir until thickened, 1-2 minutes. Remove from the heat. Add cheese; stir until melted.
Place cauliflower in a greased 13-in. x 9-in. baking dish. Pour sauce over top.
For topping, combine the bread crumbs, butter and paprika. Sprinkle over sauce. Bake, uncovered, until bubbly, 30-35 minutes.