Warm the olive oil in a large skillet over medium heat and sauté the diced red onion until softened, about 5 minutes. Incorporate the minced garlic and dried dill, cooking for an additional 5 minutes until the mixture is tender and fragrant. Set aside to cool.

Extract as much moisture as possible from the thawed spinach until reduced to roughly one cup, then add it to the skillet with the onion mixture. Stir in the parsley, green onions, lemon zest, black pepper, and crumbled feta, mixing until evenly combined.

Preheat the oven to 375°F and prepare a baking sheet by lining it with parchment paper. On a clean work surface, unroll the phyllo pastry and slice into long strips measuring 2½ to 3 inches in width. Keep the unused pastry covered with a lightly damp towel to prevent drying.

Place a single phyllo strip on the surface and lightly brush it with melted butter. Layer a second strip directly on top and repeat the brushing. Position a heaping tablespoon of filling in one corner of the strip, shaping it gently into a triangle. Fold this corner across to the opposing edge, then continue folding in a triangular motion until the end of the strip is reached.

Transfer the formed triangle onto the baking sheet and keep covered with a damp cloth while assembling the rest. Arrange triangles close together, leaving about half an inch of space between each.

Bake for 20 to 25 minutes until golden and crisp. Remove and let cool for 10 minutes before serving.