How to Make It
In a blender, combine 3 tablespoons of the olive oil with the parsley and process until smooth. Season lightly with salt. Transfer 2 tablespoons of the parsley oil to a small bowl. Add the flaked tuna, the mayonnaise, water, capers, lemon juice, anchovies, garlic and the remaining 1 tablespoon of olive oil to the blender and puree until creamy. Season the tonnato sauce with salt and pepper.
Spoon some of the tonnato sauce onto 6 plates and arrange the tomatoes on top. Season with salt and pepper. Spoon the remaining sauce on the tomatoes, then drizzle the reserved parsley oil on top and serve with bread.