Prep the veggies. (I used Thai celery for this, and even chopped the leaves to go in at the end.)
Heat the oil in a large pot on medium heat, when hot, add the onion, carrot, celery, and garlic. Cook, stirring occasionally, until the onion is transparent and tender.
Add the mushrooms and cook for a few more minutes. A note if using straw mushrooms, only slice in half lengthwise.
Mushrooms are cooked down and stirred to coat with any oil in the pot.
Add the broth and barley and stir together. I used 4 cups bean broth from chickpeas and 2 cups of water.
Bring to a boil then reduce to a simmer. Cover and simmer until barley is tender, about 50 minutes, stir occasionally, and add broth as needed. Taste and season with salt and pepper as desired. About 10 minutes before serving, add some chopped celery tops if using those, and if using Shiitakes for a nice garnish, add those and allow them to soften.
Serve. If using the Shiitakes as garnish, spoon those out and place on top of the soup.