1. Bring phyllo dough to room temperature (keep in package). Unroll and place a damp paper towel on top of sheets. Heat oven to 350 degrees .
2. Meanwhile, heat oil in a large saute pan over medium heat. Add onion and cook 5 minutes. Add spinach (in batches, if necessary). Cook 5 minutes or until just wilted. Transfer to a bowl to cool slightly. Drain excess liquid by gently pressing mixture. Roughly chop and return to bowl. Stir in eggs, cheese, mint, oregano, nutmeg, salt and pepper. Set aside.
3. Brush a 13 x 9-inch pan with a bit of the melted butter. Place a phyllo sheet on bottom of pan; brush with more melted butter. Continue layering sheets, brushing each with butter, until there is a stack of 10. Spread spinach mixture evenly on top (drain before doing so if more liquid has accumulated). Layer the remaining 10 sheets, brushing each with butter. Score spanakopita into 12 squares. Bake at 350 degrees for 45 minutes or until golden. Let stand 10 minutes, then slice into squares along scored lines.