Pat dry the cubed lamb meat and place in a large bowl with salt, pepper, and
2 tablespoons of arrowroot flour. Toss until evenly coated.
Set Instant Pot on sauté and add olive oil. When hot, sear the lamb meat in batches until golden brown on all sides, about 2–3 minutes per side. Set browned meat aside.
Leaving the sauté function on, add garlic, celery, carrot, onion, and chile flakes to the Instant Pot. Sauté veggies until softened, about 3 minutes, stirring
frequently and scraping brown edges from the bottom and sides of the pot. Add tomato paste and stir until veggies are coated. Add red wine and stir, scraping brown bits from the bottom of the pot. Add beer and continue
scraping bits. Add potatoes, lamb (plus any of its juices), bay leaf, parsley, and 2 1/2 cups of lamb bone broth to the pot. Cancel the sauté function.
Securely fasten the lid on top of the Instant Pot. Use the meat/stew function and set the time to 35 minutes. When cooking time has completed, release the pressure for 5 minutes naturally. Then cover the top of the pot with a towel and manually release any remaining pressure in the Instant Pot by carefully turning the valve to vent. Leave vent open until all the steam has released, and carefully remove the lid.
Set Instant Pot to sauté. In a small bowl, combine remaining tablespoon of the arrowroot with 1/4 cup of lamb broth. Whisk until the arrowroot has dissolved in the liquid, making a slurry. While stirring, slowly pour the slurry into the soup, stirring, until the soup has thickened, about 2 minutes.