Preheat your oven to 375°F (190°C) and lightly grease a small 6-inch oven-safe skillet or baking dish.
Finely dice the onion and mince the garlic.
Heat a non-stick pan over medium heat and sauté the onion and garlic with a splash of water for 3 minutes until translucent.
Add the fresh spinach to the pan and cook until completely wilted, then transfer to a colander and press out all excess moisture.
In a medium bowl, whisk together the egg whites, whole egg, Greek yogurt, sea salt, black pepper, and dried oregano until smooth.
Stir the cooked spinach and crumbled feta cheese into the egg mixture.
Lay one sheet of phyllo dough into the prepared baking dish, lightly brushing with a tiny bit of olive oil, and repeat with the remaining two sheets, layering them at different angles so the corners hang over the sides.
Pour the spinach and egg filling into the center of the phyllo sheets.
Gently fold the overhanging edges of the phyllo dough back toward the center, partially covering the filling.
Bake for 25 to 30 minutes until the phyllo is deeply golden and the egg center is firm to the touch.
Allow the pie to cool for 5 minutes before slicing and serving.