Preheat oven to 400°F. Lightly butter an 8-inch square baking dish or 8 cups of a standard muffin pan. Chop the spinach finely, using a food processor if desired and transfer to a mixing bowl. Stir in remaining ingredients. Pour the mixture into the prepared baking dish or muffin pan and bake in preheated oven until set, about 20 minutes for the muffin pan, 25-30 minutes for the 8-inch baking dish. Let stand for five minutes; cut large pan into 4 pieces or loosen the edges of individual cakes with a knife and turn out onto a plate. Serve warm, drizzled with marinara sauce and additional Parmesan cheese if desired.