On the bottom of a baking dish a good 1/2 cm high salt form. Place the unpeeled onions on top. Cook in the oven heated to 220 °C on the second rack from the bottom for about 45 minutes. Let the onions cool down a little bit.
In the meantime, melt the butter in a small pan. Add the flour and cook on a low heat for 1 minute, stirring constantly. Pour in the milk and cream while stirring and let it bubble up to form a thick sauce. Remove from heat and cool.
Cut the cheese into small cubes.
Score the outermost brown covering of the cooled onions precisely with scissors and loosen slightly so that it can be opened like a flower. Fish out the onion flesh except for the outermost 1-2 layers using a pointed knife and a teaspoon. Chop the flesh of 4-5 onions, use the rest elsewhere.
Mix the minced onion meat and cheese cubes into the cream sauce and season with salt and pepper. Fill the hollowed out onions with sauce and place in a buttered baking dish. Mix bread crumbs and grated cheese and sprinkle over the stuffed onions.
Meanwhile, bake the onions in the oven heated to 200 degrees on the second rack from the bottom for about 20 min.