Preheat oven to 350°F. Spray 6 wells of a nonstick muffin tin with cooking spray. Set aside.
Whisk the eggs and milk in a bowl. Divide the spinach, tomatoes, onion, and red bell pepper among the 6 wells. Pour egg mixture over the 6 wells. Top with crumbled goat cheese.
Bake or 20 minutes, or until set. Remove from oven. Let cool before removing. Store in airtight container for up to 5 days in the refrigerator. Egg muffins can also be frozen.