Preheat your oven to 375°F and line a small baking sheet with parchment paper.
Finely dice the chicken breast into small, uniform pieces to ensure even cooking and a consistent texture in the filling.
Heat a non-stick skillet over medium heat with a splash of water, then sauté the diced onion, garlic, and chicken until the meat is cooked through.
Add the fresh baby spinach to the skillet and cook until completely wilted, then transfer the mixture to a sieve and press out all excess moisture.
In a mixing bowl, combine the chicken and spinach mixture with the egg whites, crumbled feta, and fresh dill.
Lay one sheet of phyllo dough on the parchment, brush lightly with olive oil, and top with the second sheet.
Place the filling in the center of the phyllo, fold the edges inward to create a sealed rectangular parcel, and brush the top with the remaining oil.
Bake for 20-25 minutes until the pastry is golden and crisp, then let it rest for 5 minutes before slicing and serving.