In a mixing bowl, whisk together the eggs and egg whites. Fold in the spinach, tomatoes, and onion. Stir in the basil, salt, and pepper.
Coat a large ovenproof, nonstick skillet with the oil and heat the skillet over medium heat. Pour the egg mixture into the skillet and cook for 1 minute, or until the eggs start to set. Transfer the skillet to the oven and bake for 5 to 6 minutes, or until the top is golden and puffy.
Remove the frittata from the oven and let it stand for 5 minutes before cutting into wedges. Serve wedges with 1/2 cup fresh fruit.