Bring 1 1/2 litres of water to the boil. Peel the onion and add it to the hot water with the meat, bay leaf and peppercorns. Cook over medium heat for about 1 1/2 hours. Wash the lemon hot, cut into slices and add to the stock 30 minutes before the end of the cooking time.
In the meantime, soak porcini mushrooms in 1/4 litre of lukewarm water for about 1 hour. Clean, wash and cut the vegetables into small cubes. Drain the porcini and collect the mushroom water. Chop the mushrooms finely.
Heat 20 g of fat and fry vegetables and porcini mushrooms in it. Remove meat from the stock. Pour broth through a sieve and add to the vegetables. Add porcini water, bring to the boil and cook for about 15 minutes. Drain the capers.
Knead the flour and remaining fat and use it to thicken the stock. Add the capers. Season to taste with salt and pepper. Cut the meat into slices and add to the soup. Wash the parsley and, except for something to garnish, chop finely and sprinkle over the soup.
Arrange in a terrine and garnish with parsley and possibly with lemon slices.