Preheat the oven to 350°F. Grease a 12-cup muffin tin well with nonstick spray and set aside.
In a large mixing bowl, whisk the eggs until fully combined. Add the chopped red bell pepper, spinach, crumbled feta, and dried oregano, then season with salt and pepper to taste. Stir until everything is evenly mixed.
Divide the egg mixture evenly between the prepared muffin cups, filling each about ¾ full.
Bake for 18–22 minutes, or until the egg muffins are set in the center and lightly golden on top.
Let the egg muffins cool in the pan for 5 minutes, then gently remove and serve warm or at room temperature.
Grease the pan well: Egg muffins tend to stick, so use plenty of nonstick spray or butter in each cup. You can also use a silicone muffin pan so you can pop them right out.
Chop vegetables finely: Smaller pieces help the muffins cook evenly and prevent pockets of moisture.
Don’t overfill: Filling the cups about ¾ full prevents overflow and keeps the tops smooth.
Bake just until set: Over baking can make egg muffins dry or rubbery, so remove them once the centers are firm.
Expect slight deflation: Egg muffins will puff up while baking and settle slightly as they cool, this is normal.