Preheat oven to 350 and lightly coat a muffin tin with olive oil.
Heat 1 tsp olive oil in a large skillet and sauté the spinach jut until wilted. Set aside to cool.
In a mixing bowl, combine the eggs, egg whites, salt, and pepper, whisking well.
Divide the spinach, onions and feta cheese evenly among the 12 muffin cups.
Then fill each muffin cup ¾ of the way with the egg mixture. Bake for 15-20 minutes or until the egg muffins have cooked through.
Serve immediately or store in the fridge and reheat for a quick grab and go breakfast!