Preheat oven to 425 degrees. In a large bowl, lightly beat eggs. Stir in pesto, 3/4 teaspoon salt, 1/4 teaspoon pepper, and 3/4 cup mozzarella. In a 10-inch nonstick skillet, heat oil over medium-high. Add egg mixture and cook, stirring occasionally, 1 to 2 minutes. Arrange tomatoes in a single layer on top and sprinkle with remaining mozzarella.
Bake until puffed and center is just set, 10 to 12 minutes. Let sit 5 minutes; run a rubber spatula around edge and beneath frittata before transferring to a cutting board. Cut into wedges and serve.
This easy egg dish works equally well as the centerpiece of brunch or a quick dinner.