In a large pot cook rotini pasta in boiling salted water until al dente. Drain.
Cook ground turkey or beef in medium skillet until brown. Add the diced onion, salt and pepper. Mix in half of the jar of spaghetti sauce.
In a large bowl add the browned beef mixture to the drained pasta and stir in the second half of the of spaghetti sauce. Pour mixture into a 2 quart baking dish. Cover with aluminum foil.
In a preheated 425 degree F (230 degree C) oven bake for 20 minutes. Remove foil, sprinkle shredded mozzarella cheese and return to oven for 5 to 10 minutes. Serve warm.
