Peel the potatoes and cut them into chunks. Boil them in boiling salted water for 15-20 minutes. Drain and purÄe them. Add 1/4 cup diced butter and the warmed milk, stirring gently. Keep warm in the top of a double boiler. Rinse the pork caul several times in cold water. Squeeze between your hands to remove the excess water. Spread the caul on your work surface and cut into 6 equal-sized squares. Place a piece of tuna on each piece of caul and top each one with a bay leaf and a pinch of thyme. Wrap caul tightly around the tuna pieces. Dice the bacon and fry it in a little oil over a high heat. Stir the bacon and 1/2 cup Parmesan into the mashed potatoes. Pour the balsamic vinegar into the pan where the bacon was fried and boil hard until reduced by half. In a separate frying pan fry the tuna over moderate heat in a dry frying pan for 4-8 minutes, seam-side down first. Serve with the mashed potatoes. Spoon over a little of the vinegar reduction and sprinkle with the large Parmesan pieces.
