Brown the beef: In a large skillet over medium heat, crumble and cook the ground beef until well browned and no pink remains, about 8–10 minutes. Carefully spoon off most of the excess grease.

Soften the onion: Add the diced yellow onion to the skillet with the beef. Cook, stirring frequently, for 3–5 minutes until the onion begins to soften.

Load the slow cooker: Transfer the beef and onion mixture into your slow cooker. Pour in the tomato sauce and stir everything together. If you prefer a saucier consistency, stir in up to 1 cup of water.

Season and slow cook: Add salt and black pepper to taste. Cover the slow cooker and cook on LOW for 4–5 hours or on HIGH for 2–3 hours.

Add the macaroni: About 30–40 minutes before serving, stir in the dry elbow macaroni. Ensure the noodles are submerged in the liquid. If the sauce is too thick, add a splash of extra water.

Finish cooking: Cover and cook on HIGH for 25–35 minutes, stirring once halfway through. The slumgullion is ready when the macaroni is tender and fully cooked.

Serve: Give the dish a final stir, adjust seasonings if necessary, and serve hot.