1. Preheat the oven to 350°F.
2. Lightly coat a standard 12 cup muffin tin with nonstick cooking spray.
3. Divide the spinach, red bell pepper, green bell pepper, and tomatoes evenly among the muffin cups, filling each about two thirds full.
4. Whisk together the eggs, salt, oregano, and black pepper until well combined.
5. Carefully pour the egg mixture into each muffin cup, filling each about three quarters full.
6. Sprinkle the cheese evenly over the tops.
7. Bake for 24 to 28 minutes, or until the egg muffins are fully set.
8. Let cool for a few minutes, then run a butter knife around the edges of each muffin to gently loosen them.
9. Enjoy immediately, or transfer to a wire rack to cool completely before refrigerating or freezing for later.