Preheat oven to 350°F. Grease or spray a 9x13-inch casserole dish.
Start a pot of water to boil on the stove. Cook the penne or Rigatoni just until al dente. (See instructions for timing on the box, but it's usually about 9 minutes.) The pasta will finish cooking in the oven.
While the water boils: In the skillet on the stove top, cook the diced onion over medium heat until it's translucent and getting soft.
Add the ground beef to the pan and break up the meat into small pieces; cook until browned and there's no pink showing. Drain off the grease into a heat-safe container.
Remove from the stove and drain the cooked pasta into a colander in the sink. Don't rinse. Add drained pasta back to the pot it was boiled in.
Add the pan of beef mixture to the pot of cooked, drained pasta, then add sloppy joe sauce and half the cheese. Stir gently to combine.
Transfer the mixture to prepared casserole dish and top with remaining cheese.
Bake uncovered 25 minutes at 350F or until cheese melts and edges are bubbling and beginning to brown a bit.
Cool slightly before serving. Add a green salad and a roll to the plates, if desired.