Step 1: Heat olive oil in a large pot or Dutch oven over medium-high heat. Add ground beef and cook, breaking it up with a spoon, until browned. Drain any excess fat.
Step 2: Add chopped onion to the pot and cook until softened, about 5 minutes. Stir in minced garlic and cook for another minute until fragrant.
Step 3: Stir in crushed tomatoes, tomato sauce, tomato paste, beef broth, dried oregano, dried basil, red pepper flakes (if using), salt, and pepper. Bring the mixture to a simmer.
Step 4: Add the spaghetti to the simmering sauce, ensuring it is mostly submerged. You may need to gently push it down into the liquid.
Step 5: Cover the pot and reduce heat to medium-low. Cook for 15-20 minutes, stirring occasionally to prevent sticking, until the spaghetti is al dente and most of the liquid has been absorbed, creating a thick sauce.
Step 6: Remove from heat and stir in 1/2 cup grated Parmesan cheese until melted and creamy. Taste and adjust seasonings as needed.
Step 7: Serve hot, garnished with fresh parsley and extra Parmesan cheese, if desired.