If making the quicker version, mix together the melted butter, cornstarch, milk, heavy cream, spices, seasoning, and most of the cheeses and set aside. Then continue to step 5...or, start with step 2:
In a saucepan, cook the butter and cornstarch for a minute until they form a paste.
Add the warm milk and whisk until smooth and then keep cooking for a further 2-3 minutes, until thick.
Quickly add in spices, seasoning, and most of the shredded cheese, mix and switch off the heat.
Layer half the potato slices in the Instant Pot-ready dish and pour on half the cheese sauce.
Then add the remaining potatoes and top with the remaining sauce and sprinkle on the rest of the cheese. Cover the dish with foil.
Add 1 ½ cups (350ml) of water to the Instant Pot, place a trivet inside the insert, then set the potato dish on the trivet.
Place the lid on the Instant pot, turn the valve to sealing, and set to manual pressure/high pressure for 25 minutes (you will need up to 35 minutes if you use thicker potato slices).
When done, perform a quick pressure release, then remove from the Instant Pot, discard the foil and place the scalloped potatoes under the broiler for 3-5 minutes until the top is golden and crispy.