The Instant Pot, as a pressure cooker, needs at least 1 or 1 ½ cups of water, broth or other thin liquid in the bottom to produce the steam it needs to create pressure.
Things like vegetables, soups and stews can cook directly in the Instant Pot, but what about thicker or drier things? Things that would get soggy if you added all that liquid: bread pudding, lasagna, cheesecake and the like. Cooking these things directly in the Instant Pot would produce the dreaded “Burn” warning. Bummer.
That's where Pot-in-Pot (PIP) cooking comes in. PIP is when you cook the food in a smaller pot or dish suspended over the water on the trivet/rack.
Any oven-safe casserole or ramekin is safe in a pressure cooker. I've even used stainless steel mixing bowls and mason jars. Check for chips or cracks, but if it's sound, I'd say it's safe. As long as there is enough clearance around the edge to allow for steam to circulate, it's fine.
I use my Corningware and PYREX all the time in my IP. Love them. The manufacturers haven't officially endorsed their products as safe in electric pressure cookers, but almost every IP owner I know uses these dishes.
If you have a Mini (3-quart Instant Pot) AND a 6 or 8-quart, the liner from the Mini works well as a PIP container in the larger Instant Pot.
Stacking two pots is perfectly fine. I have two cheesecake pans that I use to make two lasagnas at once because... well, I love lasagna. Cheesecake, too.
When stacking, keep the cook times of the two items in mind. You don't want one lasagna that needs 20 minutes stacked with a pot of rice that only requires 4 minutes. Your rice will be mush.
Pot-in-Pot cooking sometimes takes a little longer than cooking directly in the IP stainless steel liner. If you choose the Pot-in-Pot method for a recipe that was written for cooking directly in the liner, add 1 - 5 minutes, depending on what it is. Rice might need an extra minute or two. A dense casserole, maybe five. Don't be too concerned with failure, though. You can always close the pot and add another minute or two.
So THAT, my friends, is how Pot-in-Pot cooking works.
