Preheat oven to 325ºF. Place frozen spinach in a colander and rinse with warm water, tossing, until thawed. Squeeze out excess water place on a double layer of paper towels. Use the paper towels to squeeze out more water. Repeat until the spinach has been squeezed dry. Set aside.Heat the 1 tablespoon olive oil in a skillet over medium heat. Add chopped onion and cook until softened. Add garlic and cook 1 minute more. Remove from heat. In a large bowl, whisk eggs until combined. Stir in cheeses, salt, and nutmeg. Add spinach, parsley, and cooled onion mixture. Stir until well-mixed. Unroll phyllo sheets and cover with a lightly damp dish towel. Brush oil onto the bottom and sides of a 9x13-inch baking dish. Place one sheet of phyllo on the bottom of the dish. Brush with oil. Repeat for a total of 6 sheets of phyllo. Add the spinach mixture, spreading evenly. Place 6 more layers of phyllo on top, brushing on oil between each layer. Oil the top layer. Fold any excess onto the top. Bake for 1 hour, or until the top is golden and looks crispy. Let sit for 5 minutes before serving.