Add pasta, butter, mustard, salt, pepper, and water to the pot and stir.
Lock and seal the lid. Cook at high pressure for half the pasta cooking time on the package, minus 1 minute. (So, if the directions say 6 minutes: Half is 3 minutes, minus 1 minute, means you'll cook it for 2 minutes total.)
Use a quick release of the pressure when cooking time is through. Be sure to cover the spout with a dish towel to minimize splatters.
Stir in the milk and half and half. Then, slowly stir in the cheese 1 handful at a time. Taste and adjust salt and pepper amounts, if needed. Enjoy!
Notes/Tips
This mac and cheese is best eaten fresh, but leftovers aren't too bad! Just store leftovers in the refrigerator for 3-5 days. Warm in the microwave. Add a splash of milk if it gets dry when rewarming.
If you shred your own block of cheese, it will melt more evenly than the pre-shredded kind from the store. Here's how to shred your own cheese.
I have not tested this with white or gluten-free pasta, so I'm not sure on the timing for those.