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 wate