To prepare the livers, heat a little olive oil in a medium frying pan over a medium heat. Add the onion and sweat for 2–3 minutes until translucent. Add the livers and continue to cook for 15–20 minutes, stirring occasionally. Add the Vin Santo and vinegar and some salt and pepper, then give a brief boil over a high heat. Set aside to cool. Mash with a fork, add a little more oil, transfer to a bowl and refrigerate until needed.
To prepare the tomatoes, peel them if you wish, and cut into 5 mm circular slices. Season, dress with a little oil, and set aside in a bowl.
To prepare the crostini, preheat the grill. Cut each slice of bread into four pieces and arrange on a baking sheet. Drizzle with oil, then grill until brown. Spread with either the liver or the tomatoes.