Using the SAUTE function, add the oil into the Instant Pot. Once hot, add the sliced smoked sausage, stirring occassionally, until lightly browned all over. Remove from Instant Pot.
Add the onion and saute until soft, using a wooden spoon to scrape up any of the sausage bits that are stuck to the bottom of the pan, about 4 minutes.
Next, add the celery and carrots and cook for about 1 minute. Add the garlic and cook for another minute.
Hit CANCEL. Now, add the potatoes, bay leaves, thyme, chicken stock, apple cider, salt pepper, and finally, the sautéed sausage. Cover and set Release Valve to Sealing. Select PRESSURE COOK or MANUAL or STEW on High for 4 minutes.
When pressure cooking has finished, hit CANCEL. Naturally Release for 5 minutes, and then very carefully move the Release Valve to Venting for Quick Release.
Once the valve drops, carefully remove the lid. Stir in the sauerkraut. Taste and add more salt, if desired.
Remove the bay leaves and serve at once, with whole grain mustard on the side, if desired.