Preheat your oven to 375°F and lightly grease a small 6-inch pie dish or oven-safe skillet with a touch of olive oil.
Heat the olive oil in a skillet over medium heat and sauté the finely diced yellow onion until translucent and soft.
Add the minced garlic and fresh spinach to the skillet, cooking just until the spinach is wilted, then remove from heat to cool slightly.
In a medium mixing bowl, whisk together the large eggs, egg whites, Greek yogurt, almond flour, sea salt, black pepper, and nutmeg until the batter is smooth.
Gently fold the sautéed spinach mixture and the crumbled feta cheese into the egg batter.
Pour the mixture into your prepared baking dish, spreading it evenly to the edges.
Bake for 25 to 30 minutes, or until the center is firm and the top has developed a beautiful golden-brown crust.
Allow the pie to rest for 5 minutes before slicing into wedges and serving warm.