In a large, heavy saucepan, heat a drizzle of oil over medium heat. Add the sausage, squeezing it out of its casing into the pan. Cook, breaking up the chunks, until it's browned. (You don’t have to worry about cooking it through.) Add the celery and cook for a few minutes, until the edges start to brown. Add the lentils, water or stock, salt and pepper. Cover and simmer for half an hour or so, adding extra liquid if it looks like it needs it. If you’d like it thicker, remove the lid and simmer until it’s the consistency you like.

Serve with fresh bread.