Remove the veal from the fridge one hour before preparation and let it reach room temperature. Sear the veal in butter and place in a preheated oven at 80°C (176°F) for approx. 45 minutes until it reaches a core temperature of 54°C (129°F).
Meanwhile, prepare the sauce by mixing the drained tuna (collect the drained liquid), the mascarpone, mayonnaise, and capers (save some for garnish) in a blender into a smooth sauce and flavour with pepper, a little salt, and some lemon juice.
Mix the lettuce in a bowl with a tablespoon of olive oil and a pinch of salt. Take out four plates, scoop two tablespoons of tuna sauce in the centre of each plate. Use a sharp, non-serrated knife to cut the cooled veal into thin slices along the fibre.
Sprinkle the veal with salt and pepper to taste and place the slices on the plates. Drape the lettuce over the meat and garnish using the remaining capers and some drops of sauce on the meat. Add elements to your liking, such as freshly chopped herbs (chives, basil, or parsley), chopped olives, croutons).