Get a deep pan hot and add a healthy dollop of duck fat (or oil). Firstly fry the piece of bacon skin, fat down, so it releases some of its fat into the pan, and remove; then color your bacon slices and remove; then fry your chopped onions and leeks until softened, and add the tin of tomatoes, crushing the tomatoes in your hands as you do so. Let this cook down for 20 minutes to sweeten the tomatoes, stirring to remove all the good bits of bacon that might adhere to the pan, season, remembering the bacon is salty, add two ladles of the trotter stock, and let cook for another 10 minutes. Drain the beans but keep their liquor, add to the pan, and mix with the tomato base.