Heat oven to 350° F. Spray with cooking spray or oil four 8-inch round cake pans or 9-inch pie pans. Arrange on baking sheet.

Heat the oil in a large skillet over medium-high heat. Add the onion and mushrooms; sauté until golden. Add the tomatoes; sauté until just heated through. Add the spinach; stir just until wilted. Divide among prepared pans.

In a large bowl, whisk the egg whites, and salt and pepper until frothy. Pour the egg mixture over the vegetables. Crumble the cheese over the frittatas.

Bake 30 minutes or until golden and a knife inserted in center of a frittata comes out clean. Cut each frittata into 6 pieces and garnish with chopped basil to serve.