Preheat your oven to 375°F and line a large baking sheet with parchment paper.
Place the thawed frozen spinach in a clean kitchen towel and squeeze firmly over the sink to remove as much moisture as possible.
In a medium mixing bowl, whisk together the large egg, liquid egg whites, and Greek yogurt until the mixture is smooth and well combined.
Stir in the crumbled feta cheese, finely chopped scallions, dried dill, sea salt, and black pepper.
Add the squeezed dry spinach to the bowl and mix thoroughly to ensure the herbs and cheese are evenly distributed.
Lay one sheet of phyllo dough on a clean work surface and brush very lightly with a small amount of olive oil, then place the second sheet directly on top.
Using a sharp knife or pizza cutter, slice the phyllo sheets lengthwise into three or four wide strips.
Place a generous portion of the spinach and egg mixture at the bottom end of each phyllo strip.
Fold the corner of the dough over the filling to form a triangle, then continue folding the strip like a flag until you reach the top.
Arrange the triangles on the prepared baking sheet and brush the tops lightly with the remaining olive oil.
Bake for 20 to 25 minutes until the pastry is golden brown and crispy.