Place the stainless steel inner pot inside the Instant Pot®. Press Saute and set the time to 3 Minutes Add butter and cook onion and chopped ham.
Then add the flour and cook 3 Minutes more. Then add the milk and whisk to remove any lumps. Cook until mixture thickens and lifts off the pot. Rectify salt and pepper.
Pour the mixture into a container lined with cling wrap on the bottom, and also cover the cooked mixture to prevent a hard film from forming on its surface.
Refrigerate a couple of hours or overnight. Cut into pieces of desired size.
Dredge with flour, then dip in beaten egg and then coat with breadcrumbs.
Freeze until completely firm (make sure they do not stick together).
On the Instant Vortex control panel, press Air Fry, set the time the time to 6 Minutes and the temperature to 395ºF (200ºC). Press Start.
Once Add Food appears on the display, open the basket and put in the croquettes previously sprayed with oil. Close.
When Turn Food appears on the display, open the tray, flip the croquettes, and close.
Check the croquettes; for a darker crust, add 1 or 2 Minutes more, at most. Remove and serve immediately.