In a large saucepan, bring the water and salt to a rolling boil, stir in the barley, and boil for 2 minutes. Turn down the heat to a low simmer, cover, and cook for 40 to 50 minutes, until all the water is absorbed. Uncover, remove from the heat, and fluff with a fork. Set aside to cool while preparing the rest of the salad.
Brush the tops of the mushrooms with a damp cloth to remove any grit and discard the stems. Heat the olive oil in a large pan over medium-high heat, add the mushrooms gill-side down, and sear for 2 minutes; flip and sear 1 more minute, then flip over once more. Add the wine, tamari, and oregano and bring the liquid to a boil. Partially cover the pan, reduce the heat, and braise the mushrooms until most of the liquid has been absorbed. Turn off the heat and cover to keep warm.
While the mushrooms are braising, in a large mixing bowl whisk together the dressing ingredients. Add the Brussels sprouts and massage in the dressing for 2 minutes to tenderize. Add the spring onions (scallions), cranberries, and walnuts. Stir in the warm barley.
Slice the mushroom caps into 1⁄2-inch strips. Spoon the barley mixture into serving bowls, top with mushroom strips, and garnish with chives and a few twists of black pepper.