One of the more famous gratins is with potatoes. Of course, you can modify it as you see fit. In that recipe, you add some ham, onions, and garlic to the potatoes mixture.
Cut 1 large onion and brown it
Set aside
In the tank of the instant pot put a layer of potatoes, then a layer of the ham, onion, and garlic Repeat until the ingredients are finished
Add the milk until 2/3 of the mixture is covered
Add the pepper and nutmeg
Put the instant pot in quick cooking mode 10min