Preheat your oven to 375°F (190°C) and lightly grease a small oven-safe baking dish with the extra virgin olive oil.
Finely dice the red onion and roughly chop the fresh spinach leaves.
Slice the cherry tomatoes into halves.
In a medium mixing bowl, whisk together the whole eggs and liquid egg whites until well combined and slightly frothy.
Add the dried oregano, sea salt, and black pepper to the egg mixture and whisk again.
Stir the prepared spinach, red onion, and cherry tomatoes into the egg mixture.
Pour the mixture into the greased baking dish, ensuring the vegetables are evenly distributed.
Crumble the feta cheese over the top of the egg and vegetable mixture.
Bake for 25 to 30 minutes, or until the eggs are completely set in the center and the edges are lightly golden brown.
Remove from the oven and let it rest for 5 minutes before slicing and serving.