Pat dry the cubed lamb meat and place in a large bowl with the salt, pepper, and 1 tablespoon arrowroot flour. Toss until evenly coated.
Turn the Instant Pot on the sauté function with olive oil. When hot, in batches, sear the lamb meat until golden brown on all sides. About 2-3 minutes per side. Set browned meat aside and continue cooking until all meat is finished cooking.
Add garlic, celery, carrot, onion and chili flakes (optional) to the Instant Pot (still on sauté function). Stir and sauté the veggies until softened, scraping any of the brown edges from the pot. About 3 minutes.
Once vegetables are softened, add tomato past. Stir until veggies are coated. Add red wine and stir, scraping brown bits from bottom.
Add the potato, lamb (plus any of its juices), thyme, rosemary, bay leaf and beef broth into the pot.
Securely fasten the lid on top of the Instant Pot. Hit the cancel button then hit the meat/stew function. Lower the time to 40 minutes.
When the cook time has complete, hit the 'cancel' button and release the pressure in the Instant pot manually by carefully turning the valve to "venting". Let vent until all the steam has released and carefully remove the lid.
Turn the instant pot back onto the 'sauté' function. Meanwhile, in a small bowl combine the remaining 1 tablespoon of the arrowroot with the remaining 1/4 cup of beef broth. Whisk until the arrowroot has dissolved in the liquid, making a "slurry" that will help thicken the soup.
With the sauté function on and while stirring, slowly pour the slurry into the soup, stirring, until the soup has thickened, about 2 minutes.