Okay, wow, atmospheric steam made all the difference. Thank you, Bluehen1 .

I boiled water in my kettle, poured it over the trivet.
Set the Instant Pot to 0, left the steam valve all the way open for about 5 minutes at full boil.
Turned it off, put the cheese on top, lid back on for a few minutes.

The steam was allowed to cook the burger without the high pressure (closed valve) squeezing the juices out. The patty shrunk much less, less grease and juice inside the loaf pan. Every single bite was a juicy mess. I'd call this a success.



Less shrinkage with the valve open, so it fit the bun better. Added a few bits of bacon today.


Every single bite was a hot juicy mess.