Rinse the barley under cold water.
In a large soup pan or Dutch oven, bring the 4 cups (960 ml) of water (or broth) and the barley to a boil.
Reduce the heat, cover, and simmer for 30-40 minutes, or until the barley is tender but still has a slight chew.
While the barley is cooking, prepare your vegetables. Wipe the mushrooms clean and slice them. Finely chop the large onion.
In a separate large frying pan, melt the butter over medium heat.
Add the chopped onion and sauté for 3-4 minutes until it begins to soften and turn translucent.
Add the sliced mushrooms and salt to the frying pan.
Continue to cook, stirring occasionally, until the mushrooms have released their liquid and started to brown, about 8-10 minutes.
Do not drain the flavorful liquid.
Once the barley is tender, carefully transfer the entire contents of the frying pan—the sautéed mushrooms, onions, and all their delicious juices—into the soup pan with the barley.
Stir in the soy sauce and a generous amount of freshly ground black pepper.
Bring the soup back to a gentle simmer over very low heat for at least 15 minutes to allow the flavors to meld together.
Taste and adjust seasoning if needed. The final soup should be thick, with a beautiful golden sheen.
Serve hot, adding an extra dab of butter to each bowl if desired for added richness.
Don't discard the liquid the mushrooms release while cooking! This 'mushroom liquor' is packed with umami and adds incredible depth to the soup.
Finish with a garnish of fresh chopped parsley or chives to add a burst of freshness and color that cuts through the richness of the soup.
