Preheat oven to 350℉. Grease a standard non-stick 12-slot muffin pan with cooking spray and set aside.
Heat a large non-stick skillet over medium heat. Once hot, add in oil, red pepper, green pepper, and onion. Sauté 5 to 7 minutes, or until peppers are tender. Add in spinach and mushrooms and cook for an additional 2 minutes. In the last 30 seconds, add in minced garlic. Season with salt and remove from heat.
Crack eggs/egg whites into a large 4-cup measuring cup and whisk together. Stir in cooked veggies. Pour the egg/veggie mixture evenly into the prepared muffin pan. Bake for 15 to 20 minutes, or until the tops are firm to the touch and eggs are cooked.
Cool slightly and serve immediately. Leftovers can be stored in an airtight container in the refrigerator for about 4 to 5 days or in the freezer for 3 to 4 months. To reheat, wrap egg muffins in a paper towel and microwave for 30 to 60 seconds until warm. You can also reheat these in the oven at 350oF for 5 to 10 minutes.