Preheat your oven to 375°F (190°C) and line a large baking sheet with parchment paper.
Thaw the frozen spinach completely and squeeze it very dry using a clean kitchen towel to remove all excess moisture.
In a non-stick skillet over medium heat, lightly scramble the liquid egg whites with the dried spinach until the eggs are just set but not rubbery.
Transfer the spinach-egg mixture to a mixing bowl and stir in the crumbled feta, Greek yogurt, chopped scallions, dill, nutmeg, salt, and pepper.
Lay one sheet of phyllo dough on a clean surface, lightly brush with a tiny amount of olive oil, and stack the remaining two sheets on top, oiling lightly between each layer.
Cut the stacked phyllo lengthwise into four equal strips using a sharp knife or pizza cutter.
Place a generous portion of the spinach filling at the bottom of each strip, then fold the corner over to form a triangle, continuing to fold like a flag until the end of the strip.
Arrange the triangles on the prepared baking sheet, brush the tops with the remaining oil, and bake for 18-20 minutes until deeply golden and crisp.