Brown the Meat: Place a large skillet over medium heat. Add the ground beef and cook, breaking it apart with a spatula, until completely browned (about 8-10 minutes). Drain the majority of the excess grease.

Sauté the Onion: Add the diced yellow onion to the beef in the skillet. Cook for 3-5 minutes, stirring frequently, until the onions soften and become translucent.

Prepare the Slow Cooker: Transfer the beef and onion mixture into the slow cooker insert. Pour in both cans of tomato sauce. If using water, stir it in now. Add salt and pepper, stirring well to combine.

Slow Cook: Secure the lid. Cook on LOW for 4 to 5 hours, or on HIGH for 2 to 3 hours, allowing the flavors to deepen.

Add Pasta: Approximately 35 minutes before you are ready to serve, stir the dry elbow macaroni into the slow cooker. Ensure all noodles are pushed down into the liquid so they cook evenly.

Final Simmer: Cover and cook on HIGH for 25 to 35 minutes. Check and stir halfway through the cooking time. The slumgullion is ready when the pasta is tender and has absorbed a significant amount of the flavorful tomato sauce.

Serve: Taste for seasoning, adjust if necessary, and ladle hot into bowls.