01 - Preheat oven to 350°F. Lightly grease a 12-cup muffin tin with olive oil or nonstick spray, ensuring each cup is evenly coated to prevent sticking.
02 - In a large mixing bowl, whisk together the eggs and milk until fully blended and slightly frothy, about 30 seconds of vigorous whisking.
03 - Add the chopped spinach, crumbled feta, diced red onion if using, salt, pepper, and dried oregano to the egg mixture. Stir gently until all ingredients are evenly distributed.
04 - Pour the mixture evenly into the prepared muffin cups, filling each about three-quarters full to allow room for rising during baking.
05 - Bake for 22 to 25 minutes, or until the centers are just set and the tops are lightly golden. A toothpick inserted in the center should come out clean.
06 - Let the egg bites cool in the pan for 5 minutes. Run a butter knife around the edges of each cup to loosen, then carefully remove and transfer to a serving plate or wire rack.
07 - Serve warm immediately, or let cool completely and refrigerate in an airtight container for up to 4 days. Reheat gently in the microwave for 20 to 30 seconds.