Preheat oven to 350 degrees F.
In a skillet over medium-high heat, add olive oil and sauté onion and peppers until tender.
Add garlic and cook for 1 additional minute; remove from heat.
Spray a muffin tin with nonstick cooking spray and divide vegetables evenly between the muffin cups.
Whisk together egg whites and eggs then pour into muffin cups until almost full.
Season with salt and pepper to taste.
Top each egg muffin with shredded cheese.
Bake for 20-25 minutes, or until lightly golden brown on top.