Heat the vegetable oil in a large pot or Dutch oven over medium heat.
Add the cubed beef to the pot and brown it on all sides, which should take about 5-7 minutes.
Stir in the diced onion and minced garlic, cooking until the onion becomes translucent, about 3 minutes.
Add the sliced carrots, chopped celery, and mushrooms to the pot. Cook for another 5 minutes, stirring occasionally.
Add the pearl barley, beef broth, dried thyme, bay leaf, and season with salt and pepper.
Bring the mixture to a boil, then reduce the heat and let it simmer uncovered for about 45 minutes, or until the barley is tender.
After cooking, remove the bay leaf and adjust seasoning if necessary.
Serve hot, garnished with chopped fresh parsley.