In the insert of an Instant Pot or pressure cooker, add the macaroni, water, butter, kosher salt, ground mustard and garlic salt. Cook in the pressure cooker for 5 minutes on high, then quick release. Add the evaporated milk and half of the cheeses and use a wooden spoon to vigorously stir until the cheeses have melted then repeat with the rest of the cheese, stirring until the sauce is creamy. Add any additional toppings or mix-ins below at this time. Serve warm.