Modified from a recipe by Ina Garten. We've made this a few times for dinner, and it turned out delicious as well as beautiful. The puff pastry is much easier to work with than phyllo dough, so this comes together fairly quickly.
Preheat oven to 375. Line a baking sheet with parchment (preferable) but you can use aluminum foil with a light spray of cooking oil.
Melt butter and cook onions for a few minutes, until soft. Add garlic and cook for another minute.
Beat eggs in medium bowl. Add cooked onion, spinach, feta, parmesan, bread crumbs. Add salt, pepper, & nutmeg. Mix well with fork.
Place one unfolded sheet of puff pastry on the parchment or sprayed aluminum foil. Spread the spinach mixture over the pastry, leaving a border of one inch all around the edges. Keep the spinach flat; do not pile high.
Brush the edges with the egg wash made with the last egg. Cover with the second sheet of pastry that has been slightly flattened with a rolling pin to one inch bigger than the first sheet. (This ensures all the spinach will be covered). Press edges together with fingertips or fork, making sure mixture is sealed well. Brush just the top with egg wash, careful not to drip any down the sides. Cut a few slits in the top with a sharp knife to allow steam to escape.
Bake 30-40 minutes, until golden brown. Can be served hot or at room temperature.