Best-Ever Spanakopita

Make filling: Preheat oven to 375° and line two medium baking sheets with parchment paper. Using a clean kitchen towel, squeeze all excess moisture out of spinach. Transfer to a large bowl. In a large skillet over medium heat, heat oil. Add onion and green onions and cook until softened, 5 minutes. Add garlic, oregano, red pepper flakes if using, and season with salt and pepper. Transfer mixture to bowl with spinach. Add dill, lemon zest, feta, and nutmeg to bowl with spinach and stir until all ingredients are evenly mixed and season again with salt and pepper, if needed. Remove phyllo from refrigerator and cover with a lightly damp kitchen towel. Take 1 phyllo sheet and place on a clean surface. Brush all over with melted butter, then top with another sheet of phyllo. Brush second sheet of phyllo all over with butter, then slice into 6 long, evenly sized strips (about 12"-x-3" each). Assemble spanakopita: Place a tablespoon of spinach mixture on the corner nearest you on one strip of phyllo. Fold corner over mixture to make a triangle, then continue folding (like a flag) until phyllo strip is completely folded over spinach mixture. Repeat until all filling and all phyllo is used. Place all folded triangles seam side down on prepared baking sheets and brush with butter. Bake until golden and crisp, 20 to 23 minutes. Let cool slightly before serving.