Soak the lentils overnight in cold water to cover, then drain well. Soak the bacon knuckle separately in cold water to cover. Next day drain and pat dry. Scrape the skin well.
Melt the butter in a large saucepan. Peel and chop the onion or wash and finely shred the leeks. Add to the butter and fry gently for a few moments until soft but not brown. Stir in the soaked lentils and the water. Add the bacon knuckle, bouquet garni and a seasoning of pepper. Bring up to the boil, skim and cover with a lid. Simmer gently for 1½ hours. When the lentils are quite soft, draw the pan off the heat.
Remove the bacon knuckle. Pass the soup through a sieve or vegetable mill, or purée in an electric blender, and return to the pan. Pour the milk into a basin, sift the flour over the surface and whisk until smooth – there should be no lumps. Add to the soup and stir until boiling.
Remove the rind from the bacon knuckle and scrape the meat away from the bone. Chop the meat coarsely and add to the soup. Taste and season if required. Serve with a garnish of crisply fried bread.