Press the saute feature and add butter to liner.
Once the butter is melted, add the celery, onion, and mushrooms. Saute for about 3-5 minutes, until the onions are translucent.
Add the breadcrumbs and herbs to the pot.
Add the broth to the pot.
Whisk the egg in a small bowl and pour it into the pot. Gently combine all ingredients
Press the slow cook button
Use the + or - buttons to set the time to 4 hours.
Press the adjust button until the red light is over "normal".
Place a lid on the pot. It doesn't have to be the instant pot lid, but if you use that one, ensure it is set to venting.
Half way through the cook time stir the stuffing (gently!). Add 1/2 more broth if it is too dry. If it is too wet, add in another cup of bread crumbs.
Once stuffing is cooked, press keep warm and keep in the pot until time to serve.