Step 1

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

Step 2

Melt butter in a 1 1/2-quart Dutch oven or oven-safe pot over medium heat and cook onion until lightly browned, 5 to 8 minutes. Add celery and cook until starting to soften, stirring often, about 5 more minutes. Mix barley into the vegetables and stir until coated with butter. Fold mushrooms and green pepper into barley mixture; season to taste with salt and black pepper. Pour chicken broth into barley mixture and bring to a boil; cover casserole dish.

Step 3

Bake in the preheated oven until barley is nearly tender, about 30 minutes; uncover casserole dish and bake barley until most of the liquid has been absorbed, about 15 more minutes. Adjust salt and black pepper before serving.