Who says you can't have eggs for dinner? Serve this with a simple green salad.
Preheat oven to 350 degrees. In a 10-inch ovenproof nonstick skillet, heat 2 teaspoons oil over medium heat. Cook onion, stirring often, until soft, about 5 minutes. Cool completely.
In a large bowl, whisk together eggs, egg whites, 3/4 teaspoon salt, and 2 tablespoons water. Stir in onion and 3/4 cup mozzarella.
In skillet, heat 1 tablespoon oil over medium heat. Add egg mixture; reduce heat to low, and cook until beginning to set around the edges, about 3 minutes. Transfer to oven; bake until set, about 12 minutes.
Meanwhile, heat remaining 2 teaspoons oil in a large skillet over low heat. Cook garlic until soft, 2 minutes. Add tomatoes, mint, and 1/4 teaspoon salt; simmer, breaking up tomatoes with a spoon, 5 minutes.
Spoon sauce over frittata; sprinkle with remaining 1/4 cup cheese. Return to oven; cook until cheese melts, about 1 minute.