Preheat broiler. Heat an 8-inch ovenproof skillet over medium-high heat. Coat pan with cooking spray. Add mushrooms to pan; sauté 12 minutes or until lightly browned. Place mushrooms in a medium bowl; cool slightly. Wipe pan clean with paper towels. Combine mushrooms, cheese, thyme, 1/4 teaspoon freshly ground black pepper, 1/8 teaspoon salt, egg whites, and eggs in a medium bowl, stirring well with a whisk. Heat pan over medium heat. Coat pan with cooking spray. Add mushroom mixture; cook, covered, for 3 minutes or until almost set. Broil 3 minutes or until egg is set. Cut into 4 wedges.