Slice mushrooms. Heat olive oil in a medium skillet and saute mushrooms until browned and soft. Remove from heat and set aside.
In a small saucepan, bring chicken stock to a boil and then reduce to a low simmer. Keep heat on low.
Put 2 Tbs. butter in a heavy-bottomed pot over medium heat. When the butter is melted, add the diced onion and cook, stirring occasionally, until the onion softens, 3 to 5 minutes.
Add the barley and cook, stirring occasionally, until it is glossy and coated with butter, 2 to 3 minutes. Add thyme, bay leaf, and garlic, cooking for an additional minute. Add a little salt and pepper, then the white wine. Stir and let the liquid bubble away.
Use a ladle and begin adding the stock, 1/2 cup or so at a time, stirring after each addition. When the stock is just about evaporated, add more. Keep the heat at medium to medium-high and stir frequently.
Begin tasting the barley 20 minutes after you add it; you want it to be tender but still with a tiny bit of crunch; it could take as long as 30 minutes to reach this stage. When it does, stir in the cooked mushrooms, with their juices, and at least 1/2 cup of Parmesan if you're using it. Taste, adjust the seasoning, and serve immediately.