Place the pasta, water, and 1/4 teaspoon salt in a 6-quart or larger Instant Pot or electric pressure cooker. Stir well. Set to cook on high pressure (manual) for 4 minutes (for elbow macaroni noodles) or 5 minutes (for shells). As soon as the timer goes off, release the remaining pressure.
Carefully open the lid to the Instant Pot. If it still looks very liquidy, drain off all but a thin layer of the pasta water, reserving any water you drain away. (I used an oven mitt to pick up the Instant Pot insert and drain a small amount of excess water into a liquid measuring cup with a spout.)
Stir in the butter until it melts. Then, stir in the mustard, black pepper, cayenne, nutmeg, and remaining 1/4 teaspoon salt.
Next, stir in the Greek yogurt.
Add the cheese by small handfuls (a few tablespoons at a time), stirring to completely melt between each addition. Don't rush the process or your mac and cheese will be lumpy (your patience will be rewarded!). If at any point, the pasta cools too much, turn on the Saute function for a minute or two as needed. The mac and cheese should look very creamy and saucy, but if it is thicker than you would like, add a little of the reserved pasta water until your desired consistency is reached. Taste and adjust seasoning as desired. Enjoy!
Notes
*For the smoothest, creamiest mac and cheese, shred your own cheddar from a block. Pre-shredded cheeses are coated with a powdery substance to keep them from flaking and will not melt smoothly. The fastest way to do this (it takes seconds!) is in a food processor fitted with a grater blade.
TO STORE: Refrigerate mac and cheese in an airtight storage container for up to 5 days.
TO REHEAT: Rewarm leftovers in a pot on the stovetop over medium-low heat or in the microwave.
TO FREEZE: Freeze mac and cheese in an airtight, freezer-safe storage container for up to 3 months. Let thaw overnight in the refrigerator before reheating.