In a small bowl, whisk eggs, egg whites, salt and pepper until blended. In a small nonstick skillet, heat butter over medium-high heat. Add mushrooms; cook and stir 3-4 minutes or until tender. Add spinach; cook and stir until wilted. Reduce heat to medium., Add egg mixture; cook and stir just until eggs are thickened and no liquid egg remains. Stir in cheese.