Preheat oven to 350°. Spray 12 standard muffin cups with cooking spray.
Heat large skillet over medium-high heat; spray liberally with cooking spray. Add steak and ¼ teaspoon salt; cook and stir 3 minutes or until steak is browned. With slotted spoon, transfer steak to plate. Add mushrooms; cook and stir 4 minutes or until tender. Add spinach; cook and stir 1 minute or until wilted. Remove from heat; stir in steak and cool slightly. Makes about 3 cups.
In medium bowl, whisk eggs, egg whites and ¼ teaspoon each salt and pepper.
Divide steak mixture into cups; top with egg mixture and cheese. Bake 17 minutes or until internal temperature reaches 160º. Serve cups garnished with thyme, if desired.
Serving Size 2 cups
Servings 6
* Percent Daily Values are based on a 2,000 calorie diet. Your daily value may be higher or lower depending on your calorie needs.