Press the Sauté button on the Instant Pot and add the olive oil. Add half of the beef to the pot and cook just until browned. Remove from the pan and place on a plate, then repeat with the second batch, adding the remaining tablespoon of olive oil and more as needed.

Add the onion and sauté for 3 minutes until softened, then add the garlic and sauté for 30 seconds more. Add the tomato paste and stir well.

Add the Guinness to the pan and scrape up any brown bits from the bottom. Add the stock and Worcestershire sauce and stir well to combine, then add the beef back to the pot along with any accumulated juices and stir. Add the bay leaf and thyme, then secure the lid and close the venting valve. Cook on high pressure for 30 minutes, then quick release the pressure and remove the lid when safe to do so.

Remove the lid, then remove the bay leaf and thyme sprigs. Add the carrots, celery and parsnips to the pot and secure the lid once more and close the venting valve. Cook an additional 5 minutes at High Pressure, then quick release the pressure and remove the lid when safe to do so.

Combine the cornstarch with 2 tablespoons water, stirring until smooth. Press the Sauté button and slowly stir the cornstarch slurry into the stew. Cook for about 3 minutes or until the sauce has thickened, stirring occasionally. Season with salt and pepper, to taste, and enjoy!