Set the Instant Pot to Sauté mode and add the chorizo. Stir frequently until it releases its oil and there is no pearly white fat remaining. This should take a couple of minutes. If there is lots of oil, spoon it away - you only need about 1 tbsp worth.
Turn off Sauté and add the macaroni, mustard, nutmeg and water and give it a good stir. Set the Instant Pot to manual mode and adjust the time to 4 mins. Put the lid on and set the vent to "Sealing".
When the cooking cycle has finished, do a Quick Pressure Release (QPR). There should be little or no excess water left and the pasta will be fully cooked. Press cancel and the Instant Pot will remain in Keep Warm whilst you finish the mac & cheese.
*= evaporated milk is what it says on the tin - milk cooked down until thick and sticky. Don't confuse it with condensed milk which is 50:50 milk to added sugar! I only say this as the tins can be remarkably similar and I *may* have bought the wrong one on more than one occasion!
Top Tip - if you can bare waiting a moment, put the Instant Pot inner pot in the sink and add some washing up liquid and hot water before you eat so making the clean up a lot easier!