Preheat the oven to 350°F. Use cooking spray to generously grease a non-stick 12-cup muffin tin.
Heat a large non-stick skillet over medium heat.
Add the oil, red pepper, green pepper, and onion, and cook for 5-7 minutes until the veggies are tender, stirring often.
Add in the spinach and mushrooms and cook for an additional 1 ½ minutes. Add the minced garlic and cook for an additional 30 seconds.
Season with salt and remove from the heat.
Place the egg whites into a large 4 cup measuring cup or bowl and whisk until smooth. Stir in the cooked vegetables.
Pour the mixture into the prepared muffin cups and bake for 15-30 minutes until the tops are firm to the touch and the eggs are cooked.
Cool slightly and serve.