Preheat oven to 350°F (175°C) and generously coat a 12-cup muffin tin with non-stick cooking spray.
In a large mixing bowl, whisk together the eggs, milk, salt, black pepper, and dried oregano until the mixture is uniform.
Stir the chopped spinach and diced red onion into the egg base.
Divide the mixture evenly among the 12 muffin cups, filling each approximately three-quarters full.
Distribute the crumbled feta cheese evenly across the top of each muffin cup.
Bake for 20 to 22 minutes, or until the eggs are firm to the touch and the edges are lightly golden.
Allow the muffins to cool in the pan for 5 minutes before using a silicone spatula or knife to gently release them from the tin.