Preheat oven to 400 degrees. Warm frozen spinach in a saucepan over medium heat. Drain excess liquid. Combine spinach, onion, and creamed corn in casserole dish. Melt 1 tbsp margarine and add to casserole dish. Add vinegar, salt, and black pepper. Mix ingredients together. Spread 1/2 cup of bread crumbs and 2 tbsp Parmesan cheese over top of casserole. Melt remaining margarine and drizzle over topping. Bake for 20 to 30 minutes.