Thaw spinach. Squeeze water from spinach and set aside. Preheat oven to 350. Melt butter in large skillet over medium heat. Chop onion. Add spinach and onion and saute until onion is translucent, stirring frequently, about 5 minutes. Add crabmeat and Parmesan, stirring well to blend. Transfer spinach mixture to 1-qt casserole and bake 15-20 minutes. Or transfer mixture to crockpot to keep warm, stirring occasionally. Can be served hot or cold with crackers. Makes 1/2 cup per "serving"