Preheat your oven to 375°F (190°C).
Thaw the frozen spinach completely and squeeze out every drop of excess moisture using a clean kitchen towel or fine-mesh sieve.
Mince the garlic clove and thinly slice the green onions.
In a medium skillet, heat the olive oil over medium heat and sauté the garlic and green onions for 2 minutes until fragrant.
In a large mixing bowl, combine the squeezed spinach, sautéed aromatics, crumbled feta, egg whites, whole egg, Greek yogurt, chopped dill, salt, and pepper.
Lightly grease a small oven-safe baking dish or deep ramekin.
Lay one sheet of phyllo dough into the dish, allowing the edges to hang over, then top with the second sheet.
Pour the spinach and egg mixture into the center of the phyllo dough.
Fold the overhanging phyllo edges over the top of the filling to create a rustic crust.
Bake for 30 to 35 minutes until the phyllo is golden brown and the filling is set.