Preheat oven to 375°F. In medium bowl, whisk eggs with 1/4 teaspoon salt. In a large oven-safe nonstick skillet, heat 2 teaspoons oil over medium heat. Add eggs and cook until edges are set, about 4 minutes. Top evenly with spinach, peppers, and cheeses. Bake until eggs are cooked through and the cheese has melted, about 10 minutes. Cool 2 minutes and slice.
In a large bowl, toss tomatoes with basil, 1/8 teaspoon salt, and 1 teaspoon olive oil. Serve with the frittata.