In a large pot, brown the ground beef and onions over medium heat until almost all the pink is gone and the onions are translucent. If there is an excess amount of oil in the bottom of the pan, you can tilt the pan and spoon out the oil to discard. If there is only a little bit of oil, don't worry about it. The amount of fat at the bottom of the pan will depend entirely on the ground beef that you choose to use and its fat content. Stir in the minced garlic and cook for 1 minute.
Stir in the diced tomatoes and marinara sauce. Cook, stirring on occasion, until boiling. Add the sugar, onion powder, Italian seasoning, thyme, salt and pepper; stir to combine.
Break the spaghetti in half (or leave it whole, if desired) and add it to the pot along with the broth. Push the noodles down to submerge them in the sauce. Bring to a boil, then cover and reduce the heat to simmer, stirring every 5 minutes, for 15 minutes or until the pasta is done and the sauce is thicker.
Serve individual servings topped with Parmesan cheese. Add parsley garnish, if desired.