Rinse the beef leg slices and add to a large saucepan with 2 liters of cold water. Bring to a boil over high heat. Skim off the protein foam that rises to the surface. Peel and halve the onion. Heat a non-stick frying pan over medium heat and roast the onion halves without oil for 5 minutes until browned. Add the roasted onion, bay leaves, and salt to the saucepan. Reduce heat to low and simmer the beef for 90 minutes.
While the beef simmers, wash and clean the soup vegetables and beans, peeling if necessary. Peel and wash the potatoes. Cut the beans, soup vegetables, and potatoes into bite-sized pieces.
After 60 minutes of cooking, add the soup vegetables and potatoes to the broth. After 75 minutes total cooking time, add the beans. Once 90 minutes total cooking time has elapsed, remove the beef from the saucepan and let cool for 5 minutes. Remove the meat from the bone and cut into cubes. Strain the broth and discard the onion and bay leaves. Return the beef cubes to the saucepan with the broth. Heat over medium heat for 3 minutes until warmed through. Season with salt and pepper and serve.