Heat olive oil in a large skillet and place over medium heat. Saute onions and garlic for 3 minutes until soft. Add the spinach, season with salt and pepper, and continue to saute until the spinach is limp, about 2 minutes. Add lemon juice, remove from heat and place in a colander, and squeeze out excess liquid. Set aside to cool. The filling needs to be cool and dry to prevent the phyllo from becoming soggy. In a medium bowl, beat the eggs with feta, coriander, and nutmeg. Season, then fold in the cooled spinach mixture until well blended.
Preheat oven to 350 degrees F, brush 2 baking sheets with some melted butter. Unroll the phyllo dough and lay a sheet flat on a work surface. Take care to keep the phyllo covered with a damp, not wet, towel as you work to prevent drying out and becoming brittle. Brush the sheet with melted butter, then sprinkle evenly with some oregano and chives. Repeat with 2 more sheets of phyllo, stacking on top of each other. With a sharp knife or pizza cutter, cut the sheets lengthwise into thirds to form 2 1/2-inch strips. Do this with all the sheets of dough.
Place a heaping teaspoon of filling near 1 corner of the layered phyllo strip. Fold the end at an angle over the filling to form a triangle. Continue to fold the triangle along the strip until you reach the end, like folding up a flag. Brush the top with butter and dust with Parmesan, place on prepared baking sheet, and cover while preparing the remaining pastries. Repeat until all the filling and phyllo strips are used up. Bake for 20 to 30 minutes until the triangles are crisp and golden. Serve hot, warm or cold.