In a large pot or Dutch oven, brown the ground beef over medium heat. Drain off any excess grease.
Add the chopped onion and minced garlic to the pot. Cook until the onion is softened, about 5-7 minutes.
Stir in the crushed tomatoes, beef broth, and tomato paste. Add the dried oregano, dried basil, red pepper flakes (if using), salt, and black pepper.
Bring the sauce to a simmer, then reduce the heat to low, cover, and cook for at least 45 minutes, stirring occasionally. For a richer flavor, you can simmer it for up to 1.5 hours.
While the sauce is simmering, cook the spaghetti according to package directions. Drain the spaghetti.
Serve the bolognese sauce over the cooked spaghetti. Garnish with grated Parmesan cheese.
Notes
This recipe can be made ahead of time and reheated. The flavors deepen overnight.