Brown the beef: In a large pot or Dutch oven over medium-high heat, brown the ground beef with the onion, breaking it up as it cooks, about 6-8 minutes. Drain off the excess grease.
Add the garlic: Stir in the garlic and cook for 1 minute until fragrant.
Build the sauce: Stir in the diced tomatoes, tomato sauce, beef broth, Italian seasoning, salt, and pepper. Bring to a simmer.
Cook the macaroni in the pot: Stir in the elbow macaroni. Cover and simmer for 12-15 minutes, stirring occasionally, until the pasta is tender and the sauce has thickened around it.
Finish and serve: Stir in the shredded cheddar until melted, and serve warm.
Notes
Cook the macaroni right in the sauce instead of boiling it separately, it soaks up more flavor and the starch it releases thickens the sauce.