Add the butter, cornstarch, heavy cream, milk, nutmeg, paprika, pepper and salt into a bowl and whisk until well combined.
Then mix in 1 1/2 cups (1.5 190g) of cheddar and 2 tbsp of Parmesan and switch off the heat.
Layer half the potatoes slices in the Instant Pot-ready dish and pour on half the cheese sauce.
Then add the remaining potato rounds and top with the remaining cheese sauce and sprinkle on the rest of the cheeses.
Cover the potatoes with foil and place on a trivet.
Add 1 1/2 cups (350ml) of water to the Instant Pot and place the dish on the trivet in the Instant Pot.
Close pot, set valve to sealing, manual, high pressure and timer to 25 mins. (You will need up to 35 mins if you use thicker slices.)
Let cook till done and then do a quick pressure release.
Remove the potatoes from the Instant Pot, discard the foil and place the potatoes under the grill / broiler for 3-5 mins until the top is golden and crispy.
Top with some herbs and serve.