Boil the cauliflower for 4 min. in salted water, drain them and put in an oiled tray. Melt the butter on the stove and add the flour to it, braise 2-3 min.
Slowly add the heated milk while stirring nonstop, to break down any lumps, until thickened for about 2 min.
Remove from the stove and add 1/2 the cheese and parmesan. Pour this mixture over the cauliflower.
Combine the walnuts, breadcrumbs, remaining cheese and parmesan and sprinkle over the cauliflower and sauce. Bake in a preheated oven until golden.