First wash the cauliflower and separate small florets. Boil these in salted water for about 15 minutes. Catch the cooking water when draining.
Melt the butter in a saucepan and add the flour with a whisk while stirring constantly. Then add approx. 300-400 ml of the cooking water while stirring (depending on how thin you want the sauce) and bring to the boil.
Cut the ham into small pieces and mix with the cauliflower in a baking dish. Pour the sauce over it and sprinkle with the cheese. Place in the oven preheated to 200 degrees for about 10 minutes until the cheese has run.