Preheat your oven to 375°F (190°C).
In a large non-stick skillet, sauté the finely chopped yellow onion and minced garlic until translucent and fragrant.
Add the fresh spinach to the skillet in batches, cooking until completely wilted.
Place the cooked spinach mixture into a clean kitchen towel or fine-mesh strainer and squeeze firmly to remove every drop of excess moisture.
In a medium mixing bowl, whisk together the large egg, liquid egg whites, and plain Greek yogurt until smooth.
Fold in the crumbled feta cheese, chopped fresh dill, sea salt, and black pepper.
Stir the squeezed spinach and onion mixture into the egg and yogurt base until well combined.
Lightly brush a small oven-safe baking dish with the extra virgin olive oil.
Layer the two sheets of phyllo dough into the dish, allowing the edges to hang over the sides.
Pour the spinach and egg filling into the center of the phyllo-lined dish.
Gently fold the overhanging phyllo edges back over the filling to create a rustic crust.
Bake for 30 to 35 minutes, or until the filling is set and the phyllo pastry is deeply golden and crisp.