Heat the vegetable broth in a saucepan and keep it warm on low heat.

In a large skillet, heat olive oil over medium heat. Add the chopped onion and sauté until translucent, about 3-4 minutes.

Add the minced garlic and sliced mushrooms. Cook until the mushrooms are tender and browned, about 5-6 minutes. Remove half of the mushrooms and set aside for garnish.

Add the pearl barley to the skillet and stir to coat with the oil and vegetables. Toast the barley for 2 minutes until slightly fragrant.

Pour in the white wine and stir until it is mostly absorbed by the barley, about 2 minutes.

Begin adding the warm broth, one ladle at a time, stirring frequently. Allow each ladle of broth to be absorbed before adding the next. Continue this process for about 30-35 minutes until the barley is tender and creamy.

Stir in the butter, Parmesan cheese, and thyme. Season with salt and black pepper to taste.

Garnish with the reserved mushrooms and fresh parsley before serving.