Heat a large heavy-bottomed pot over medium-high. Add the beef (and oil, if lean) and cook, breaking it up, until browned, 6 to 8 minutes. Drain all but 1 tablespoon of fat.

Add the onion and garlic; cook until softened, 3 to 4 minutes. Stir in the macaroni and toast for 1 minute.

Pour in the broth, milk, salt, and pepper. Bring to a simmer, then cover, reduce heat to low, and cook 10 to 12 minutes, stirring once, until pasta is tender.

Remove from heat. Fold in the shredded cheddar a handful at a time until smooth. Taste and adjust seasoning. Spoon into bowls and serve immediately.