The night before you plant to serve the soup, place the barley in a large bowl with water to cover by 2 to 3 inches. Set to soak overnight.
The following day, drain the barley, transfer it to large pot, and add enough water to cover by 2 to 3 inches. Bring to a boil, reduce to a simmer and cook until the barley is al dente, about 40 minutes. Drain the barley.
In a large, dry saucepan, sauté the onion with a pinch of salt over medium heat until golden brown and softened, 8 to 10 minutes. Add the olive oil and when it is very hot, stir in the mushrooms and sauté them until they are nicely browned, 3 to 4 minutes.
Add the drained barley to the saucepan and stir until it is well combined with the onion and mushrooms. Deglaze the saucepan with the wine, allowing the alcohol to evaporate. Pour the stock into the saucepan and simmer the soup on low heat for about 35 minutes. Add the parsley, salt and pepper. Continue to cook for 10 minutes, or until the soup is cooked to your liking and all the ingredients are blended together nicely.