Preheat your oven to 375°F (190°C).
Sauté the diced yellow onion and minced garlic in a non-stick skillet until translucent and fragrant.
Add the fresh spinach to the skillet in batches, cooking until completely wilted.
Transfer the cooked spinach to a colander or clean kitchen towel and squeeze out every drop of excess liquid to prevent a soggy crust.
In a large mixing bowl, whisk together the egg, liquid egg whites, crumbled feta cheese, chopped dill, sea salt, and black pepper.
Stir the squeezed spinach and sautéed onion mixture into the egg and feta base until well combined.
Lightly brush a small oven-safe baking dish with a portion of the olive oil.
Lay one sheet of phyllo dough into the dish, allowing the edges to hang over, and brush lightly with olive oil; repeat with the second sheet.
Pour the spinach and egg mixture into the phyllo-lined dish and fold the overhanging dough edges toward the center.
Bake for 25 to 30 minutes until the filling is set and the phyllo is golden brown and crisp.