Preheat oven to 400Trim cauliflower florets from the stalk. Discard stalk.
Cut florets into 1-2 ince pieces. Steam the cauliflower for about 10 minutes until firm but tender.
Spread the florets out onto a paper towel to dry while making the cheese sauce
In a medium sauce pan, melt the butter over medium high heat, add the flour and whisk to combine. Cook for about 1-2 minutes to get rid of the flour taste.
Add the mustard powder, pinch of cayenne, black pepper and stir to combine.
Drizzle the milk in a steady stream whisking the whole time so that the mixture stays creamy.Season with salt and bring to a simmer, stirring constantly.When mixture thickens, add the cheese one handful at a time. Taste the sauce and adjust the salt/pepper as needed.
Spread the cauliflower florets into a 2 quart baking dish (or iron skillet), spoon the sauce over the top and sprinkle about 2 tbs of cheese over the top.