Heat ¼ cup (1/2 stick) butter in a large deep skillet over medium heat until the foam subsides.
Add 1 1/3 cups finely chopped onion and cook, stirring, until tender but not brown, about 7 minutes.
Stir in 8 ounces of shiitake mushrooms, stems removed, caps diced, and cook until softened.
Add 1 cup pearl barley and stir until glazed with butter.
Add the following and cook, stirring, until the liquid is absorbed, about 3 minutes:
2/3 cup dry white wine
1 tablespoon mashed or finely minced garlic
½ teaspoon salt
½ teaspoon black pepper
Meanwhile, bring 8 cups of chicken broth to a boil, then reduce the heat to a gentle simmer.
Stir 2 cups of the broth into the barley. Cook at a moderate to brisk simmer, stirring occasionally, until the broth is almost absorbed, 8 to 9 minutes. Add the remaining stock, ½ cup at a time, allowing each addition to be absorbed before adding the next and stirring often; it will take 4–5 minutes of each addition to be absorbed and a total of 45–55 minutes for the barley to become tender. If you run out of stock before the barley is done, finish cooking with hot water.
If serving as a main course, stir in ½ to 1 cup grated Parmesan cheese.
Garnish with 2-3 tablespoons chopped parsley or 1 to 2 teaspoons chopped thyme.
This can be made up to 4 days ahead. Let cool completely, then cover and refrigerate. Reheat in a skillet over low heat, adding a little water and stirring frequently.