Heat 2 teaspoons of olive oil in a large saucepan over medium-high heat.

Add the chicken, 1/2 teaspoon salt, and 1/2 teaspoon ground pepper.

Cook the chicken until it is cooked through, about 5 minutes.

Transfer the cooked chicken to a plate and keep warm.

Heat the remaining 2 teaspoons of olive oil in the same saucepan.

Add the chopped onion, carrots, and celery to the saucepan.

Cook, stirring occasionally, until the vegetables soften, about 5 minutes.

Add the minced garlic, dried thyme, and the remaining 1/2 teaspoon salt and 1/2 teaspoon ground pepper.

Cook for 1 minute, stirring continuously.

Stir in the pearl barley, chicken broth, and diced tomatoes.

Bring the mixture to a boil, then reduce the heat to medium-low.

Cover the saucepan and simmer until the barley is tender, about 50-60 minutes.

Stir in the cooked chicken and minced flat-leaf parsley.

Serve the chicken barley stew hot.