Poach or pan-sear the chicken breast until cooked through, then shred it into bite-sized pieces.
Pre-heat your oven to 375°F and line a baking sheet with parchment paper.
Heat the olive oil in a large skillet over medium heat and sauté the diced onion and minced garlic until they are soft and fragrant.
Add the fresh spinach to the skillet and cook until completely wilted, then transfer the mixture to a fine-mesh colander and press firmly to remove all excess moisture.
In a large mixing bowl, combine the squeezed spinach, shredded chicken, crumbled feta cheese, egg, chopped dill, nutmeg, sea salt, and black pepper.
Lay one sheet of phyllo dough on the prepared baking sheet, mist it lightly with water, and place the second sheet directly on top.
Mound the spinach and chicken filling into the center of the phyllo sheets, leaving a two-inch border around the edges.
Fold the edges of the phyllo dough inward over the filling to create a rustic, open-faced pie.
Bake the pie for 22 to 25 minutes until the pastry is golden brown and the filling is set.