Preheat oven to 400 degrees Fahrenheit (200 degrees Celsius).

In a saucepan or large skillet, melt the butter. Whisk in the flour and cook until starting to bubble, about two minutes. Add garlic and stir, then add milk and whisk well to combine. Whisk slowly until bubbles form around the edges, then slowly add the cheeses in batches, allowing the cheese to melt before adding more.

Season with one teaspoon salt, pepper, and one and a half teaspoons paprika. Whisk well until very smooth.

Grease a 9 x 13-inch (23 x 33-centimeter) dish with cooking spray and add the cauliflower. Season the cauliflower with half a teaspoon salt. Pour the cheese sauce over the cauliflower and mix gently to combine.

Combine panko crumbs, half a teaspoon paprika, and cayenne pepper and sprinkle over the top. Bake uncovered until bubbly, about 20 minutes. Reheat uncovered.