½ pound salt pork, cut into 1-inch cubes
2 tablespoons butter
1 tablespoon vegetable oil
2 onions, finely chopped
2 leeks, including some of the green, finely slice
1 stalk celery, thinly sliced
1 carrot, peeled and thinly sliced
2 tomatoes, peeled, seeded, chopped
1 tablespoon tomato paste
salt and freshly ground black pepper
1 cup dried lentils
2 quarts white Stock or beef bouillon
1 bouquet garni

Bring 4 cups of water to a boil in a saucepan.  Add the cubed salt pork and cook for 5 minutes over medium heat, then drain and dry thoroughly on paper towels. 

Heat the butter and oil in a large, heavy flameproof casserole. Add the pork cubes and cook until lightly browned, then spoon off all but 2 tablespoons of fat from the casserole. Add the onions and leeks and cook until soft and lightly browned. Add the celery, carrot, tomatoes, tomato paste, salt, and pepper. Bring the mixture to a boil and cook for 5 minutes, stirring frequently.  Add the lentils, stock, bouquet garni, and optional piece of beef. Cover the casserole and simmer the soup for 2 hours, skimming it several times. 

Cool the soup and degrease it carefully. Taste, correct the seasoning, and discard the bouquet garni. 

Just before serving, reheat the soup and serve, accompanied by black bread and bowl of sweet butter.