Preheat the oven to 350°F. Grease a muffin pan with cooking spray.
In a large bowl, whisk together the eggs, nonfat milk and 1/2 teaspoon pepper. Stir in the spinach, tomatoes and onions.
Divide the mixture evenly between the 12 muffin pan cups and bake the muffins for 20 to 25 minutes, or until the egg is fully cooked. Remove the muffins from the oven and let them cool for 5 minutes in the pan then use a knife to loosen the muffins from the cups.
Top each muffin with sliced avocado, a dollop of salsa and a sprinkling of cheese then serve.
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