Preheat your oven to 375°F (190°C).
Heat a small non-stick skillet over medium heat and add a tiny drop of olive oil. Sauté the diced yellow onion and minced garlic until translucent and fragrant.
Add the fresh spinach to the skillet and cook just until wilted. Remove from heat and squeeze out any excess moisture using a clean kitchen towel or paper towels.
In a medium mixing bowl, combine the wilted spinach, diced cooked chicken breast, crumbled feta cheese, sea salt, black pepper, and fresh dill.
Lightly beat the egg and fold it into the spinach and chicken mixture until well combined.
Lay one sheet of phyllo dough on a clean surface and lightly brush with olive oil. Place the second sheet on top.
Place the chicken and spinach filling in the center of the phyllo sheets. Fold the sides over the filling to create a rectangular parcel, brushing the edges with a little more oil to seal.
Place the pie on a parchment-lined baking sheet, seam-side down.
Bake for 20-25 minutes, or until the pastry is golden-brown and crisp.
Allow to cool for 5 minutes before slicing and serving.