Thaw frozen spinach completely and squeeze out all excess water until very dry.

In a large skillet, heat olive oil over medium heat. Add chopped onion and sauté until softened and translucent, about 5 minutes. Stir in minced garlic and cook for 1 more minute until fragrant. Remove from heat and let the mixture cool slightly.

In a large mixing bowl, combine the squeezed spinach, cooled onion and garlic mixture, crumbled feta cheese, lightly beaten egg, chopped fresh parsley, nutmeg, and black pepper. Mix thoroughly until all ingredients are well combined.

Preheat your oven to 375°F (190°C). Line a large baking sheet with parchment paper.

Carefully unroll the phyllo dough. Keep the unused phyllo sheets covered with a damp kitchen towel to prevent them from drying out.

Melt the unsalted butter in a small bowl. Take one sheet of phyllo dough, lay it flat, and lightly brush its entire surface with melted butter. Fold the phyllo sheet lengthwise into a strip about 2-3 inches wide.

Place approximately one tablespoon of the spinach-feta filling at one end of the folded phyllo strip. Fold one corner over the filling to form a triangle shape. Continue folding the strip like a flag, maintaining the triangle shape, until you reach the end of the strip.

Place the finished spanakopita triangles seam-side down on the prepared baking sheet. Lightly brush the tops of the triangles with more melted butter.

Bake for 20-25 minutes, or until the phyllo is golden brown and delightfully crispy. Serve warm as an appetizer or side dish.