In a large, heavy saucepan or Dutch oven, saute the bacon over moderate heat until crisp and brown. Remove the bacon from the pan and set aside. Add onion, celery, carrot, garlic and marjoram and cook for about 5 minutes, or until the onion is translucent. Add the lentils, chicken stock and bay leaf, and bring just to a boil. Reduce heat to low and simmer until the lentils are tender. About 30 minutes. Taste and adjust the seasoning. If the soup becomes too thick during cooking, add more chicken stock. Garnish with bacon.