Heat a deep 12-inch 5-quart saute pan over medium-high heat. Brown beef and onion for 7 to 8 minutes, breaking the meat finely and cooking it to 160°F; spoon off excess fat.
Stir in garlic and paprika for 30 seconds. Add macaroni, broth, salt and pepper and bring to a simmer.
Cook uncovered over medium-low heat for 10 to 12 minutes, stirring and scraping the bottom every 2 minutes. Add reserved water 2 tablespoons at a time only if the pan dries before the pasta is tender.
Stir in evaporated milk and mustard and simmer gently for 1 minute; keep a little fluid sauce around the pasta.
Remove from heat and stir in American cheese, then cheddar, until smooth. Rest for 2 minutes and serve while the macaroni is loose and glossy.