Bring 3 cups water and barley to a boil in a pot. Reduce heat to medium-low, cover, and simmer until barley is tripled in volume, 30 to 40 minutes.
Heat oil in a skillet over medium-high heat. Saute onion and garlic until onions are starting to soften, about 3 minutes. Add mushrooms, sprinkle with salt, and saute until mushrooms have released their liquid and are well browned, about 5 minutes.
Mix balsamic vinegar with 1 tablespoon water in a bowl; pour into the skillet with the mushroom mixture. Cook until liquid is mostly evaporated, about 2 minutes. Add the barley and cook, stirring gently, until flavors combine, 2 to 3 minutes. Add more water if needed.