Preheat the oven to 375F, spray a 9x9 baking dish with some cooking spray, spray it well so the egg mixture does not stick.
Heat up a skillet over medium-low heat, add in the olive oil. Once hot, add the chopped onions and cook for 2-3 minutes. Add in the spinach and stir, cook until the spinach is wilted, about 2 minutes. Remove from the stove and let cool.
In a bowl whisk the eggs, add in the milk, salt, pepper, and oregano, stir well.
Add the crumbled feta cheese and the cooled spinach mixture to the eggs and lightly mix to combine it all together.
Pour the egg mixture evenly into the pan. Bake for 25-30 minutes until the eggs are set and firm in the middle. Remove and let cool for a few minutes.
Cut the egg mixture into 9 equal squares or you can cut into circles like I did (for pictures) Place each piece on the english muffin, wrap with foil and freeze.
They can be frozen for up to 3 months, let thaw before reheating, you can use the microwave, about 45 seconds or you can pop them in the oven/toaster oven, 350F for 10 minutes. *Be sure to remove foil before using the microwave.
*Note- if adding any butter, mayonnaise or cheese, I like to add this just before I eat them.