Step 1: Prepare the tomatoes. Wash and thoroughly dry all tomatoes. For larger tomatoes, slice them into 1/4-inch thick rounds or wedges. Halve or quarter smaller cherry or grape tomatoes. Arrange the prepared tomatoes artfully on a large serving platter, mixing colors and shapes for visual appeal.
Step 2: Make the tonnato sauce. In a food processor or blender, combine the drained tuna, mayonnaise, 2 tablespoons of drained capers, anchovy fillets (if using), fresh lemon juice, Dijon mustard, and 1 tablespoon of the reserved tuna oil or a good quality extra virgin olive oil. Blend until the mixture is completely smooth and creamy. If the sauce is too thick, add a tiny splash of water or more lemon juice, a teaspoon at a time, until it reaches a pourable, yet thick, consistency. Taste the sauce and adjust seasoning with salt and freshly ground black pepper as needed.
Step 3: Assemble the platter. Spoon or generously drizzle the prepared tonnato sauce over the arranged tomatoes on the platter. Ensure an even coating, but don't worry about perfection – a rustic drizzle often looks best for a Tomato Tonnato Platter.
Step 4: Garnish and chill. Sprinkle generously with fresh chopped parsley or basil. For an extra touch, you can add a few more capers and a final drizzle of high-quality extra virgin olive oil. Chill the Tomato Tonnato Platter in the refrigerator for at least 30 minutes before serving to allow the flavors to meld beautifully.
Step 5: Serve. Present your stunning Tomato Tonnato Platter as an elegant appetizer or a light meal. Serve with crusty bread or focaccia for scooping up the delicious sauce and tomatoes.