1. In a large pot, melt butter over medium-high. Add onion, garlic, and thyme; season with salt and pepper. Saute until onion is softened, about 6 minutes. Add button mushrooms and cook until beginning to soften, about 8 minutes. Add broth, 4 cups water, and shiitake mushrooms. Bring to a boil and cook until all mushrooms are tender, 12 to 15 minutes. Add barley, reduce to a simmer, and cover. Cook until barley is tender, about 12 minutes; season with salt and pepper.