Heat the oil in a large heavy-based casserole and sauté the onions, carrots and celery until they soften and brown. If using the pork belly, sauté it with the vegetables.
Slice the tripe into strips about 5 mm wide, although you’ll find the length will vary.
Add the tripe to the casserole with the wine. Simmer until the wine has evaporated, then add the tomatoes and seasonings and simmer gently with the lid askew for 11 ⁄2 hours, or until the tripe is creamy and soft.
Add the borlotti beans, if you’re using them, towards the end of the cooking time.
Make the polenta following my polenta recipe and serve with the tripe.