Brown the beef: In a large pot or Dutch oven, brown the cubed sirloin over medium-high heat. Remove the beef and set aside.

Sauté vegetables: Add the diced celery, carrot, and onion to the pot and cook until softened, about 5-7 minutes.

Add mushrooms and garlic: Add the diced oyster, shiitake, and crimini mushrooms, and minced garlic to the pot. Cook until the mushrooms release their liquid and begin to brown, about 5-7 minutes.

Combine ingredients: Return the browned beef to the pot. Add the diced potato, beef stock, rehydrated shiitake mushrooms (if using), pearl barley, thyme, and pepper.

Simmer: Bring the soup to a boil, then reduce heat to low, cover, and simmer for at least 2 hours, or until the barley and potatoes are tender. Stir occasionally.

Adjust seasoning: Taste and adjust seasoning as needed. Serve hot.