Soak peas in cold water overnight. Drain on a sieve, rinse with cold water and cook together with the ham bone in 1 3/4 litre of water, covered, for about 1 3/4 hours.
Onions peel and roughly chop. Clean and wash the carrots, quarter them lengthwise and cut them into slices. Clean, wash and cut leek into 1 cm wide rings. Heat the fat in a pot. Sauté the onion cubes in it until golden brown.
Add the carrot pieces and leek rings, also sauté briefly. Deglaze with the broth and season with marjoram. Cover and cook over medium heat for about 10 minutes. Remove the ham bone from the peas.
Add the vegetables with liquid to the peas. Season to taste with salt and pepper. Cut the sausages into slices and add them. Cook everything together for another 5 minutes. Stir in between.
Wash and finely chop the parsley. Spread the soup on preheated plates and sprinkle with chopped parsley.