Preheat the oven to 350°F. Grease a muffin pan with cooking spray. In a large bowl, whisk together the egg whites, spinach, tomatoes, mushrooms, cheese, chives, 1/2 teaspoon salt and 1/4 teaspoon pepper. Divide the mixture evenly among the 12 muffin pan cups.
Bake the muffins for 20 to 25 minutes, or until the egg whites are fully cooked. 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 diced 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 avocado and serve.