In a large Dutch oven or casserole, combine the veal, chicken stock, wine, onions, carrots, celery, rosemary, bay leaves, and peppercorns. Add water as needed to cover the meat and bring to a boil. Reduce the heat to a simmer and cover tightly. Simmer, turning once, until the meat is tender and an instant-read thermometer registers 145 to 150 degrees F, about 50 minutes to 1 hour.
Transfer the veal to a platter and let cool. Strain and reserve the cooking broth. Wrap the veal tightly and refrigerate until well chilled (up to 24 hours).
To make the sauce, combine the tuna, anchovies, capers, white wine vinegar, lemon juice, and lemon zest in a blender or food processor and blend to a very smooth texture, about 2 minutes. Add the mayonnaise and process to blend thoroughly. Add 1 tablespoon of the reserved cooking broth. Add more, if desired, to taste or for a thinner consistency. Refrigerate in a tightly sealed container for at least 2 and up to 24 hours. (The sauce tastes best if made 1 day in advance and allowed to rest overnight).
To serve, thinly slice the meat and arrange on a platter. Drizzle the sauce over each slice, season generously with the kosher salt and black pepper. Garnish by sprinkling with additional capers and chopped parsley. Arrange the lemon slices around the platter and serve.
Yield: 4 to 6 appetizer servings