Boil a large, salted pot of water for the pasta and cook it 2 minutes less than package directions suggest. Once it's done, drain it and add it back to the same pot it was cooked in.
Meanwhile, add the meat to a skillet, along with the chopped onion. Cook over medium-high heat, breaking the meat up as you go along, for about 10 minutes or until browned and cooked through.
Add the skillet mixture to the pot that has the pasta in it, and then pour in the marinara sauce. Toss until everything is coated (this is easiest to do with tongs or two large spoons).