In a large soup pan, melt the butter over medium-high heat.
Add the chopped onion and sauté for 3-4 minutes until it begins to soften and become translucent.
Add the sliced mushrooms and a pinch of salt.
Cook, stirring occasionally, for 8-10 minutes, until the mushrooms have released their liquid and started to turn golden brown.
Stir in the rinsed barley and toast for one minute until it becomes fragrant.
Pour in the vegetable broth and soy sauce.
Bring the mixture to a boil.
Once boiling, reduce the heat to low, cover the pan, and let it simmer for 45-60 minutes, or until the barley is tender and chewy.
Add more broth or water if the soup becomes too thick.
Season with freshly ground black pepper and adjust salt if needed.
Serve hot, garnished with fresh parsley.
While water works, using a quality vegetable broth will create a much richer, more complex soup base. A mushroom or umami-rich broth is even better!