1 Heat the oil in a 10-inch nonstick skillet over medium-high heat. Add the onion and mushrooms; sauté until golden.2 Add the tomatoes; sauté until tender. Add the spinach; stir just until wilted. Season with salt and pepper.3 Whisk the egg whites, and salt and pepper to taste until frothy.4 Pour the egg mixture over the hot vegetables in the skillet; stir gently to combine.5 Reduce the heat to medium-low. Cook without stirring until the eggs are set on the bottom, about 5 minutes.6 Sprinkle the cheese over the frittata. Broil until the cheese melts and the top is set, 2 to 3 minutes. Garnish with chopped basil.