Preheat the oven to 350 degrees. Lightly coat a baking dish with cooking spray. In a nonstick frying pan, cook ground beef and onion until the meat is browned and the onion is translucent and drain well. Fill a large pot 3/4 full with water and bring to a boil. Add the pasta and cook until al dente, 10 minutes. Drain the pasta thoroughly. Add the cooked pasta and spaghetti sauce to the meat and onions. Stir to mix evenly. Spoon the mixture into the prepared baking dish. Bake until bubbly, about 25 to 35 minutes. Divide the pasta among individual plates. Sprinkle each with 1 tbsp. Parmesan cheese and serve immediately.