Preheat the oven to 350 degrees F (175 degrees C).

Melt butter in a 1 1/2-quart Dutch oven or oven-safe pot over medium heat. Add onion; cook until lightly browned, 5 to 8 minutes. Add celery; cook and stir until starting to soften, about 5 minutes. Stir in barley until coated with butter. Stir in mushrooms and bell pepper; season with salt and black pepper. Add chicken broth; bring to a boil. Cover Dutch oven.

Bake in the preheated oven until barley is nearly tender, about 30 minutes. Uncover Dutch oven; continue baking until most of the liquid has been absorbed, about 15 minutes more. Taste; adjust seasonings before serving.