Peel or clean, wash and coarsely chop the soup greens. Wash the meat, dab dry. Halve the onion and roast the cut surfaces in a pot without fat. Brown the meat and soup vegetables.
Add a good 1 1/2 l of water. Wash the thyme and add to the meat with the bay leaf. Bring to the boil and simmer at low heat for about 2 hours. Skim in between.
Remove the meat from the stock and let it cool down a little. Sieve the stock and measure 1 l. Remove meat from the bone and cut into bite-sized pieces.
Wash the sage and cut into strips. Peel garlic and cut into thin slices. Heat the oil in a pot. Sauté sage and garlic in it. Add meat stock, bring to the boil. Heat the meat in the stock.
Preheat oven (electric cooker: 225°C/circulating air: 200°C/gas: level 4). Grate cheese and sprinkle on the slices of bread. Bake in a hot oven for 3-4 minutes until golden brown.
Remove soup from the stove. Whisk egg yolk and cream, stir into the hot soup (do not boil any more!). Season to taste with salt and pepper. Serve the garlic soup with the crostini.