Clean the cauliflower, cut off the stalk below the florets, then cut crosswise at the cut. Pre-cook the cauliflower in salted water (approx. 5-7 minutes) and drain well.
Knead the beef and pork with the egg, squeezed bun, salt, pepper, nutmeg and chopped parsley and stuff between the branches of the cauliflower.
Sauté chopped onions in fat, add drained canned tomatoes and simmer briefly. Put the tomato mixture in an ovenproof dish. Place the filled cauliflower on top and cover the tin in the preheated oven. Steam the cauliflower at 250 degrees for 30 minutes. In the meantime, toast the breadcrumbs in butter and spread over the cauliflower before serving.
Serve cauliflower with white bread and possibly a green salad.