I've never worked with phyllo dough, but it was much easier than I expected. These are baking in the oven right now. Smell really, really good.
Adapted from Annie's Eats (based on what I had in the house)
2 tbsp. olive oil
1/2 cup chopped yellow onion
1 (10-oz.) package frozen chopped spinach, thawed and drained
2 eggs, lightly beaten
2 tbsp. freshly grated Parmesan cheese
2 tbsp. plain dry bread crumbs
1/4 tsp. grated nutmeg
1 tsp. kosher salt
1/2 tsp. ground black pepper
1 cup small-crumbled feta cheese
12 sheets frozen phyllo dough, defrosted
6 tbsp. unsalted butter, melted
Preheat the oven to 375 degrees F.
Heat the olive oil in a medium sauté pan, add the onion, and cook for 5 minutes over medium-low heat. Meanwhile, add the thawed and drained spinach to a large mixing bowl. When the onions are done, add them to the bowl. Mix in the eggs, Parmesan cheese, 1 1/2 tablespoons bread crumbs, nutmeg, salt and pepper. Gently fold in the feta.
Place one sheet of phyllo dough flat on a work surface. Brush the dough lightly with butter. Working quickly, place another sheet of phyllo on top of the first, and brush it lightly with butter. Continue this process until you have a stack of 4 sheets of phyllo. Cut the stack of phyllo in half lengthwise (I found a pizza cutter useful for this step). Place 1/3 cup of the spinach filling on the shorter end and roll the phyllo up diagonally as if folding a flag. Then fold the triangle of phyllo over straight and then diagonally again. Continue folding the first diagonally and then straight until you reach the end of the sheet. The filling should be totally enclosed. Continue assembling phyllo layers and folding the filling until all of the filling is used. Place on a sheet pan, seam sides down. Brush the tops with melted butter, sprinkle with kosher salt and bake for 30-35 minutes, until the phyllo is browned and crisp. Serve hot.