Frittata with Tuna and Tomatoes

Name: Frittata with Tuna and Tomatoes
Ingredients: 6 large eggs, beaten, at room temperature; Two 5-ounce cans tuna packed in olive oil, drained and flaked into 1/4 to 1/2-inch pieces, such as Flott; 1/4 cup milk, at room temperature; 3 tablespoons chopped fresh flat-leaf parsley; 1 tablespoon anchovy paste; 1/2 teaspoon kosher salt; 1/2 teaspoon freshly ground black pepper; 1 tablespoon extra-virgin olive oil; 1 tablespoon unsalted butter, at room temperature; 2 Roma tomatoes, seeded and chopped; 1 cup (8 ounces) mascarpone, at room temperature; Zest of 1 small lemon