The stock may be made from meat or vegetables. Soak the peas for twelve hours in cold water, and then drain them. Put them into a saucepan with the cold stock and bring to the boil. Add the onion and simmer until the peas are soft. Put through a sieve, and mix the purée and stock with the potato flour. Season with salt and pepper and chopped marjoram. Add pieces of the bacon and sliced sausages, and simmer for ten minutes. Stir in a little sour cream just before serving.