How to Make It
Chop 1 of the onions into 1-inch pieces, and chop 2 of the celery stalks into 1/2-inch pieces; set aside. Cut remaining onion and 2 celery stalks in half crosswise; set aside.
Preheat Instant Pot (or other programmable pressure cooker) to the “Sauté” setting. Sprinkle beef with salt and 1⁄2 tsp. of the pepper. Add oil to preheated pot. Add beef; cook, turning occasionally, until browned on all sides, about 10 minutes. Remove from pot; set aside. Add mushrooms; cook, stirring often, until tender and browned, about 5 minutes. Remove from pot; set aside. Add the chopped onion and celery pieces; cook, stirring often, until softened, about 5 minutes. Add garlic; cook, stirring constantly, until fragrant, about 1 minute. Return beef and mushrooms to pot. Stir in broth, carrot, tied thyme sprigs, the halved onion and celery, and remaining 1⁄4 tsp. pepper.
Close lid on the Instant Pot, making sure valve is in the sealed position. Select “Manual,” and set the pot to 30 minutes on high pressure.
Meanwhile, fill a medium saucepan with water; add barley. Bring to a boil over high; cook until tender, about 10 minutes. Drain.
When the cook time on the Instant Pot ends, carefully flip the valve to do a quick release. Once the pressure has come down, remove the lid.
Remove and discard carrot, onion and celery halves, and tied thyme sprigs. Skim fat from top of soup. Stir in cooked barley. Sprinkle with thyme leaves.