Step 1

Grate the cheddar and American cheeses on a box grater. Do not use pre-shredded cheese. If the American cheese is in slices, cut it into thin strips. Set aside.

Step 2

Spray the crock of a 6 to 8-quart slow cooker with nonstick cooking spray. Add the uncooked macaroni, butter, evaporated milk, whole milk, dry mustard, and salt to the slow cooker. Stir to combine. Add the cheeses and stir to combine.

Step 3

Cover and cook on low for 1 hour. Stir well, cover again, and cook until the pasta is done to your liking - about 1 additional hour (2 hours total). Switch the slow cooker to warm and serve immediately. The exact cook time will vary based on the specific slow cooker. Add salt and pepper (or additional seasoning) to taste before serving.As the dish sits, it will thicken more and the pasta will continue to soften. Gradually add warm milk to thin the sauce if it gets too thick.