Gently fry the pork belly, lardons or bacon in the butter in a large casserole until crisp and brown all over. Remove with a slotted spoon and set aside.
Add all the remaining ingredients except the sausage to the pan with 3 tablespoons water. Cover and cook on a low-medium heat for 25 minutes.
Stir in the sausage and cook for 10 minutes more. Season with lots of black pepper (it shouldn’t need much salt) and top with the bacon to serve.