01 - Preheat oven to 350°F. Lightly grease a 12-cup muffin tin with olive oil or nonstick spray, coating each cup thoroughly.
02 - In a mixing bowl, whisk together the eggs and milk until completely blended and slightly frothy.
03 - Stir in the chopped spinach, crumbled feta, diced red onion, salt, pepper, and dried oregano until evenly distributed throughout the mixture.
04 - Evenly pour the egg mixture into the prepared muffin cups, filling each about three-quarters full to allow for rising.
05 - Bake for 22 to 25 minutes, or until the centers are just set and the tops are lightly golden.
06 - Let cool in the pan for 5 minutes, then run a knife around the edges of each egg bite to gently release and remove them.
07 - Serve warm immediately, or let cool completely and refrigerate in an airtight container for up to 4 days.