Peel the potatoes and cut into bite-sized pieces, boil or steam them. Also cut the cauliflower into bite-sized pieces and boil or stew. Meanwhile, let the mixed vegetables thaw, remove the herb butter and mix with the sauce (cold), season if necessary.
Put a layer of the sauce in a large casserole dish so that the base is covered, then add the hot potatoes and cauliflower and the mixed vegetables, distribute everything a little colorful. Then pour the rest of the sauce over it and cover well with cheese at the end.
Bake in the oven at 180 ° C for another 20 - 30 minutes, or until the cheese is golden.