Blanch leg slices in boiling water for about 2 minutes, drain and rinse with cold water. Cut onion in half and brown in a pan without fat for about 4 minutes, remove. Clean or peel the soup greens, wash and cut into large pieces. Put leg slices, soup greens and onion halves into a pot. Fill up with 2 litres of water, bring to the boil and simmer at medium heat for about 2 1/2 hours. Skim off any foam immediately with a skimmer, repeat this process occasionally
Take out leg slices and let them cool down. Pour the stock through a sieve (better: sieve with inserted, moistened passing cloth) into another pot. Bring the broth to the boil again and let it simmer for 20-25 minutes (so that the broth boils down a bit and the taste is intensified)
Clean and wash the leek and cut into thin rings. Heat the oil in a pot and fry the leek for about 2 minutes. Deglaze with wine and simmer for 3-4 minutes. Add stock, bring to the boil and simmer for about 8 minutes
In the meantime, cut the bread into slices about 1 cm thick, cover with cheese and place on a baking tray lined with baking paper. Bake in a preheated oven (electric cooker: 200 °C/ circulating air: 175 °C/ gas: see manufacturer) for 3-5 minutes, remove from oven. Remove the leg slices of meat from the bones and skin and pull them apart. Heat up in the soup approx. 5 minutes before the end of the cooking time. Season to taste with salt and pepper. Pour the soup into 4 soup plates and serve with the bread au gratin