In a large pot over medium-high heat, melt the butter. Once melted, mix in the diced onions, carrots, and celery. Sauté until the vegetables are tender, approximately five minutes.
Stir in the sliced mushrooms and season with salt and pepper. Allow the mixture to cook until the mushrooms soften, which takes about 5 to 8 minutes.
Next, pour the chicken broth into the pot. Rinse the pearl barley under cold water and add it to the pot along with the bay leaves.
Bring the soup to a rolling boil, then lower the heat and let it simmer until the barley becomes tender, roughly 40 to 50 minutes. Ladle into bowls, serve warm, and savor the flavors!