Preheat your oven to 375°F (190°C) and lightly grease a small oven-safe baking dish.
In a skillet over medium heat, add the olive oil and sauté the diced yellow onion and ground turkey until the turkey is fully browned and cooked through.
Stir in the minced garlic and fresh spinach, cooking for 2 minutes until the spinach is wilted and any excess moisture has evaporated from the pan.
In a medium mixing bowl, whisk together the egg, Greek yogurt, crumbled feta cheese, sea salt, black pepper, and dried oregano until well combined.
Fold the cooked turkey and spinach mixture into the egg and yogurt custard.
Lay the phyllo sheets into the prepared baking dish, allowing the edges to hang over the sides, and lightly brush or mist with a tiny bit of water or oil between sheets.
Pour the turkey and egg filling into the center of the phyllo sheets, then gently fold the overhanging edges of the pastry back over the top of the filling.
Bake for 25 to 30 minutes, or until the custard is set in the center and the phyllo pastry is a deep golden brown.