Place an oven rack in the center of the oven and preheat to 400 degrees F.
For the frittata: Mix together the eggs, tuna, milk, parsley, anchovy paste, salt and pepper in a large bowl. Heat the oil and butter over medium-high heat in an oven-safe, 10-inch nonstick skillet. Add the egg mixture and cook, without stirring, for 5 minutes. Arrange the tomatoes on top of the egg mixture and cook until a crust forms on the underside and the sides of the mixture have set, about 3 minutes.
Place the skillet in the oven and bake until the center is set, for 6 to 8 minutes. Using a heat-resistant spatula, loosen the sides of the frittata and slide onto a serving plate.
For the topping: Mix together the cheese, salt and lemon zest in a medium bowl.
Cut the frittata into wedges and serve with a dollop of the topping.