Set the Dutch oven over the heat and warm the oil.
Add the beef and onion and cook until the meat is browned and onion is softened, about 20 minutes.
Stir in the garlic and cook for 1 minute.
Pour in the beef broth and marinara sauce, cover, and bring to a simmer.
Add the spaghetti (you may need to break it to make it fit), and stir gently to submerge it in the sauce. Cover and cook at a gentle simmer, stirring occasionally to prevent it from sticking, until the pasta is tender, about 17 minutes.
Season to taste with salt and pepper. Serve with grated parmesan cheese.
Notes
Keep it saucy--The spaghetti will continue to soak up liquid once you remove it from the fire so it should still be quite saucy when you're finished cooking. Add extra broth or water if needed to ensure a saucy consistency.
Protect your oven--Since you're cooking in a Dutch oven outside, the cooking time can vary due to outside temperature and wind. If you're cooking on a very windy day, try to protect the Dutch oven with a barrier, such as a tarp.