Make sure the stainless steel insert is in your Instant Pot.
Prepare carrots and potatoes, by cutting them into large chunks. Dice celery, onion and slice mushrooms. Mince garlic. Season meat with salt and pepper.
Press “saute” setting on your Instant Pot. Wait 2 minutes, then add olive oil. Place stew meat in the Instant Pot. Let it brown for 4 to 5 minutes. Remove onto a plate.
To your Instant Pot, add diced onion, minced garlic and sliced mushrooms. Stir, trying to scrape the bits from searing the meat from the bottom of the pot. Saute veggies until the onions are almost translucent. Add 1/4 cup of beef stock and deglaze the bottom of the pot. Press "cancel/off" button.
Add potatoes, carrots, celery and meat back into the pot. Stir. Add Guinness stout beer (if using) and remaining beef stock. Add brown sugar and tomato paste. Stir everything together. Place bay leaf and fresh thyme on top.
Close lid, set valve to "sealing" position. Make sure the Instant Pot is set to cook on HIGH pressure. Press the "manual" or "pressure cook" button. Set timer to 30 minutes. It will take about 10 minutes for the IP to come to pressure.
For best results, let the Instant Pot release pressure naturally. If you are in a hurry, wait 7 to 10 minutes, then switch valve to "venting".
Whisk cornstarch with water.
Open the lid carefully away from your face. Add cornstarch mixture. Stir well.