Coat the inside of a 5- or 6-quart slow cooker with butter or nonstick cooking spray. This helps prevent sticking and makes serving and cleanup easier.

Add the uncooked elbow macaroni, melted butter, evaporated milk, whole milk, sour cream, salt, black pepper, and optional mustard to the slow cooker. Stir until the pasta is coated and the dairy mixture is distributed throughout.

Stir in the shredded sharp cheddar and mozzarella or Monterey Jack. Reserve a small handful of cheese for sprinkling over the top if desired.

Cover and cook on LOW for about 2 to 3 hours, stirring once or twice during cooking. Begin checking at the 2-hour mark because slow cookers vary in temperature.

When the macaroni is tender and the cheese sauce is creamy, switch the slow cooker to WARM. Serve immediately, since extended cooking can make the pasta too soft and cause the sauce to become oily.