In a pot add in the beef stock, miso, porcini powder, and thyme. Heat until simmering then lower the heat all the way to keep warm.
In a wide pan on low-medium heat melt 3 tbsp of the butter. Once melted add the shallots and garlic, and cook until soft and fragrant.
Add the barley and toast for 3-4 minutes.
Add the first round of broth. Depending on the pan add broth until it just covers the risotto. (could be 1 cup to 1 1/2 cups). Give it a quick mix, a shake of the pan to ensure all grains are covered and cook until rice absorbs the majority of the broth. Mix about 2-3 times per broth add.
*** you can add a lid to the pan between mixes and adds, this will speed up the cooking process since barley takes a while longer to cook. Keep and eye on it though and don't let the grains stick.
Add more broth and repeat above steps. The barley will take close if not all of the 6 cups of stock. Start tasting at the 30-35 minute mark. Is the barley still hard? Add more broth. Is it not loose enough? Add more broth. Also a note on salting, the miso adds a lot of sodium to this dish. Only salt if needed.
While the barley is cooking start the mushrooms. In a wide skillet over medium heat add the 3 tbsp of the remaining butter. Once hot layer in the mushrooms in a single layer. Cook for 4-5 minutes, or until golden brown along the edges and flip. Lightly salt and cook for another 4-5 minutes. Once done place on the side until the risotto is finished.
When the barley is done, it should be loose and al dente, remove it from the heat. Add in the parmesan cheese and mushroom and toss. Serve immediately.