In a large Dutch oven or deep skillet over medium heat, combine ground Italian sausage, diced onion, and minced garlic. Cook until the sausage is browned and the onions are softened, about 5–8 minutes. Drain any excess fat.

Stir in marinara sauce, diced tomatoes, and chicken broth. Season with salt and pepper, then bring to a gentle simmer.

Break the spaghetti noodles in half and add to the pot, pressing them into the sauce. Stir gently to submerge the pasta, then cover and cook for 15–20 minutes, stirring halfway through, until the noodles are tender.

Once the spaghetti is al dente, remove from heat and let sit, covered, for 2 minutes. If the sauce seems too thick, add a splash of reserved pasta water.

Garnish with grated Parmesan cheese and chopped parsley. Serve hot and enjoy a cozy, flavorful meal.