Rinse the lampredotto under cold running water and place it in a pot filled up to 3/4of water with a carrot, the celery, the tomatoes, the onion and a handful of coarse salt. Bring to the boil and let it simmer on low heat for at least an hour.
Meanwhile make the salsa verde. Soak the bread in water and white vinegar, squeeze it well and crumble it with your hands. Chop the parsley with a knife along with the hardboiled egg, a clove of garlic, the capers and the crumbled bread. Put all the chopped ingredients in a bowl and pour in a generous dash of extra virgin olive oil, stir well and season with salt and pepper.
To make the panino, remove the lampredotto from the broth and slice it thinly. Open the panino and fill it with the lampredotto, season with plenty of salt and ground black pepper and a generous spoonful of salsa verde.