1. Preheat oven to 350 degrees F. Lightly spray a 12-cup muffin tin with nonstick oil spray.
2. In a large bowl, whisk together eggs and onion. Season with salt and pepper, to taste.
3. Add egg mixture halfway up into each tin.
4. Divide the three filling combinations into 4 muffin cups each.
5. Bake for 15-20 minutes, until set.
6. Let cool slightly, then serve OR store in an airtight container in the refrigerator for up to 4 days and reheat when ready to serve.