Heat the oven to 350 F. 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. Drain well and set aside.
Fill a large pot 3/4 full with water and bring to a boil. Add the pasta and cook until al dente, 10 to 12 minutes, or according to the package directions. 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 macaroni among individual plates. Sprinkle each with 1 tablespoon Parmesan cheese. Serve immediately.