Heat oil in a large pot over medium-high heat and brown chicken for 5 minutes, turning; transfer to a plate before it finishes cooking.
Add onion, mushrooms, carrots and celery to the pot and cook for 8 minutes until softened, scraping up browned bits.
Stir in barley, broth, thyme, salt and pepper. Simmer gently for 25 minutes.
Return the chicken and its juices to the pot and simmer for 20 to 25 minutes more until the barley is tender and the chicken reaches 165°F.
Rest off the heat for 5 minutes, stir and ladle into bowls while plenty of broth remains around the grains.