Melt the butter in a large pot set over medium-high heat. Add the onions, carrots, and celery and sauté until the veggies are softened- about five minutes.
Add the mushrooms and season to taste with salt and pepper. Cook until the mushrooms are softened — about 5-8 minutes.
Pour the broth into the pot. Rinse the pearl barley and add it to the soup along with the bay leaves.
Bring the soup to a boil, then reduce the heat to low and cook until the barley is tender — about 40-50 minutes. Serve the soup warm and enjoy!