Peel the onions properly. Spread the salt evenly on the bottom of a baking dish and place the onions inside. Meanwhile, cook in the oven heated to 200 degrees on the lowest shelf for forty-five to sixty minutes until soft, the exact time depends on the size and quality of the onions. Cool.
Using a sharp-edged spoon or a baller, carefully scoop out the onions except for the three outermost layers. Chop the removed flesh from about half of the onions as small as possible.
Melt the butter and briefly soften the flour. Mix the milk and cream and add while stirring. Bring to the boil, it should form a thick sauce. Cool slightly.
Cut the cheese into small cubes. Mix with the finely chopped onion meat and season with salt and pepper.
Place the hollowed out onions in a baking dish. Fill with the cheese and onion sauce. Mix the breadcrumbs and cheese and sprinkle over the onions.
Meanwhile, bake the onions in the oven heated to 200 °C for about twenty minutes.
Serve with fried or parsley potatoes.