Boil the macaroni in salted water for 5 minutes, making sure it is very al dente. Drain and rinse the pasta in a colander. (Alternatively, you can use raw pasta, simply increase the cooking time to 4-5 hours)
Meanwhile, spray the inner crock of a 6-quart slow cooker with cooking spray, or grease with butter.
Add the al dente macaroni, butter, milk, cream, 1 ½ cups of cheddar, the gouda and mozzarella, flour, mustard, onion powder, and salt and pepper to taste to the crock. Stir well, cover, and cook on low for 2 ½ to 3 hours, stirring occasionally. Starting at 2 hours, try tasting for doneness.
In the last 15 to 20 minutes, sprinkle the remaining cheddar on top of the pasta. Before serving, taste and add any additional seasonings. Serve immediately or keep crockpot on warm setting for up to 2 hours.
Notes
Pasta: Macaroni elbows work best, but corkscrew pasta and mini shells are also good options. Do not use thin pasta or gluten-free as it will become mushy.Uncooked pasta: Rinse it before adding it to the crockpot. Cooking time will be 4 to 5 hours. You still need to stir the pasta occasionally. Cheese: Do not use pre-shredded cheese (it doesn't melt as smoothly). Shred the cheese yourself.Store in an airtight container in the refrigerator for up to 3 days. This dish is best when served the day it is made.