Cook the cauliflower roses in salted water until al dente. Peel the jacket potato and cut into bite-sized pieces.
For the sauce, melt the butter and stir in the flour. Add the milk and vegetable soup, stirring continuously, and bring to a gentle simmer. Season with the spices and fold in half of the cheese.
Place the potatoes, cauliflower florets and ham in a gratin dish, pour the sauce over them and sprinkle with the remaining cheese. Bake the casserole in the heated oven at 220 °C until au gratin.