Line slow cooker with aluminum foil caller and coat completely with vegetable oil spray.
Bring evaporated milk, condensed soup, water, 1tsp salt, mustard and cayenne to simmer in large pot.
Slowly whisk in cheddar and monterey jack until completely melted, then stir in macaroni.
Transfer mixture to prepared slow cooker. Cover and cook until macaroni is tender, about 2 hours on high
Remove foil collar, stir pasta adding hot water as needed to loosen sauce consistency. Season with salt and pepper to taste.
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