Place pasta and cold water (or chicken broth) into pressure cooker. Do your best to create an even layer of pasta so its all mostly submerged. Add pressure cooker lid, lock into place, seal valve and cook 5 minutes under high pressure on "Beans/Chili" mode.
While pasta is cooking, mix butter and flour together to create a paste.
Quick release pressure once cooking cycle has completed and remove lid. Switch to Brown/Saute function. Pour in evaporated milk and the butter/flour mixture. Continuously stir until sauce thickens.
Sprinkle in grated cheeses, little by little, until it all has been incorporated. Add in splashes of milk as needed if macaroni and cheese gets too thick. (I probably used 1 cup total as I was stirring in the cheese.)
Stir in nutmeg, taste and adjust seasonings as desired. Serve immediately.
Notes
I used a combination of grated white cheddar, gruyere and parmesan cheeses that I bought in blocks (by weight) and grated myself. You'll want to select and use 12 oz. of easily meltable cheeses for a smooth and creamy macaroni and cheese, and 4 oz of other, more flavored cheeses for best results.Good meltable cheeses could include: