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 minced garlic until soft and fragrant.
Add the fresh spinach to the skillet and cook just until wilted, then remove from heat and let cool slightly.
In a medium bowl, whisk together the egg whites, sea salt, black pepper, nutmeg, and chopped fresh dill.
Stir the sautéed spinach mixture and the crumbled feta cheese into the egg white mixture.
Lay one sheet of phyllo dough into the bottom of the baking dish, allowing the edges to hang over, and lightly brush or spray with a touch of oil.
Pour the spinach and egg mixture over the first sheet, then top with the second sheet of phyllo, folding the edges inward to seal.
Bake for 25-30 minutes until the phyllo is golden-brown and the egg filling is set and firm to the touch.