Preheat your oven to 375°F (190°C) and lightly grease a 6-inch oven-safe skillet or small ceramic baking dish.
Heat the olive oil in a non-stick pan over medium heat, then add the diced red onion and sauté for 3-4 minutes until translucent.
Stir in the minced garlic and fresh spinach, cooking for about 2 minutes until the spinach is fully wilted.
Place the wilted spinach mixture into a fine-mesh sieve and press firmly with a spoon to squeeze out all excess liquid.
In a medium mixing bowl, whisk together the whole eggs, egg whites, and Greek yogurt until the mixture is completely smooth and pale yellow.
Fold the squeezed spinach and onions into the egg mixture, then stir in the sea salt, black pepper, and ground nutmeg.
Pour the mixture into your prepared baking dish and sprinkle the crumbled feta cheese evenly across the top.
Bake for 25 to 30 minutes, or until the edges are golden and the center is just set with a slight jiggle.
Remove from the oven and let the bake rest for 5 minutes before slicing to allow the creamy custard texture to firm up.