Add pasta to the inner pot of the pressure cooker.
Add enough water cover pasta by 1 inch. Depending on the shape of the pasta, it usually takes 3-5 cups of water.
Lock lid in place, and turn pressure release valve to SEALING.
Cook on MANUAL/HIGH pressure for 3-4 minutes.
Carefully turn pressure release valve to VENTING.
Unlock lid, and drain pasta.
Return pasta to inner pot.
Add milk, salt, pepper, and dry mustard and stir to combine.
Press CANCEL.
Stir in cheese, and continue stirring until cheese has melted and is combined.
Stir in diced ham and frozen peas until combined.
Notes
Pasta: I like to use cavatappi. If you use another type of pasta, you may find that you need to adjust the cooking time.
Evaporated milk: You can use regular, low-fat, or fat-free.
Mustard: If you don't have dry mustard, Dijon mustard makes a good substitute. I use about a teaspoon or so if I'm using Dijon mustard.
Cheese: I've used a number of combinations of cheeses. Feel free to use the types of shredded cheese that you like.
Peas: I don't thaw the peas before using. I recommend checking for chunks of ice before adding the peas so that you don't end up with watery mac and cheese.
Cooking time: For larger pasta, like cavatappi, cook for 4 minutes. For smaller pasta, like elbow macaroni, cook for 3 minutes.