Heat a large soup pot or Dutch oven over medium-high heat. Add the beef and onion; sauté until the meat is no longer pink and the onion is translucent, about 5-7 minutes. Add the garlic and cook, stirring constantly, for 1 more minute. Drain off the excess grease. Stir in the jar of marinara sauce, broth, tomato sauce, brown sugar, basil, oregano, salt, pepper, and red pepper flakes.