Heat the olive oil in a large pot or Dutch oven over medium-high heat.
Add the beef cubes and brown them on all sides for about 5-7 minutes. Remove the beef and set aside.
In the same pot, add the diced onion, carrots, and celery. Sauté for about 5 minutes until the vegetables are softened.
Stir in the minced garlic and sliced mushrooms, cooking for another 2-3 minutes.
Return the browned beef to the pot. Add the pearl barley, beef broth, bay leaf, thyme, salt, and pepper. Bring to a boil.
Once boiling, reduce the heat to low and cover. Let the soup simmer for about 1 hour.
After 1 hour, check the barley for tenderness; if it's still firm, continue simmering for an additional 15-30 minutes as necessary.
If desired, stir in the frozen peas during the last 5 minutes of cooking.
Remove the bay leaf and taste for seasoning, adjusting salt and pepper if needed.
Serve hot, garnished with fresh parsley.