Brown the beef and onion. Add the ground beef and diced onion to a large pot or deep skillet over medium heat. Cook, breaking the beef apart as it browns, until the meat is fully cooked and the onion is softened. Drain excess grease if needed.

Add the garlic and seasoning. Stir in the minced garlic and Italian seasoning. Cook for 1–2 minutes, stirring frequently, until fragrant.

Add the sauce and liquid. Pour in the marinara sauce and water or beef broth. Stir until everything is well combined and bring the mixture to a gentle simmer.

Add the pasta. Stir the uncooked pasta shells directly into the sauce, making sure they are mostly submerged in the liquid.

Cook the shells. Cover and simmer for about 12–14 minutes, stirring every few minutes to prevent the pasta from sticking. Continue cooking until the shells are al dente. Add a splash more broth or water if the liquid is absorbed before the pasta is tender.

Make the sauce creamy. Reduce the heat to low and stir in the heavy cream until the sauce is smooth and creamy.

Season and rest. Season with salt and pepper to taste. Remove from the heat and let the pasta rest for 2–3 minutes so the sauce can thicken.

Serve. Garnish with fresh parsley and freshly grated Parmesan cheese, if desired. Serve warm.