Preheat the oven to 350°F. Grease a muffin pan with cooking spray.
Divide the spinach, cherry tomatoes, cheddar cheese and chives equally among the muffin cups.
Pour the egg whites into the muffin cups, filling each full.
Bake the muffins until the egg whites are fully cooked, 20 to 25 minutes. Remove the muffins from the oven and let them cool for 5 minutes in the pan before using a knife to loosen the muffins from the cups.
Top each muffin with mashed avocado and serve immediately, or if freezing, transfer the fully cooled muffins to a sealable plastic bag and store them in the freezer. To re-heat: Microwave the muffins for 45 seconds or until thawed, then top them with the mashed avocado and serve.