This is the quick version of the Italian classic: instead of poaching a veal eye of round yourself, you use thinly sliced roast veal from the butcher, leaving only the sauce to be made.
First, make the tonnato sauce. Place the drained tuna, egg yolk, two anchovy fillets, mustard, and one tablespoon of lemon juice in a tall beaker and blend until smooth with a hand blender. While blending, pour in the olive oil in a thin, steady stream until you have a sauce with the consistency of pourable yogurt. Too thick? Stir in a teaspoon of cold water. The sauce needs to flow: it is draped over the meat, and a stiff sauce served in dollops makes the dish heavy instead of elegant.
Taste and season with pepper and extra lemon juice if needed; salt is rarely necessary due to the anchovies.
Arrange the slices of roast veal overlapping on a large flat plate and spoon the sauce in a wide strip down the center, leaving the edges of the meat visible.
Scatter the capers and the remaining chopped anchovy over the top. Finish with the arugula in the center, a crack of black pepper, and a lemon slice.