Place veal into a pot. Rinse and peel soup vegetables, chop. Peel onion and quarter. Add onion together with soup vegetables, thyme, peppercorns and 2 teaspoons of salt to the meat. Bring to a boil and simmer, covered, for 1 hour.
Remove from heat and let veal cool in the pot.
Drain tuna and mash. Gradually add mayonnaise to tuna and season with capers, some capers liquid, salt and pepper.
Cut meat into thin slices. Arrange meat slices with tuna sauce on plates. Quarter cherry tomatoes and arrange next to meat, serve.