Preheat the oven to 350 degrees. Melt the butter in a large skillet over medium high heat. Add the onions, carrot, celery, mushrooms, barley, thyme, salt and pepper and saute, stirring frequently, until the mushrooms begin to brown, about 10 minutes.
Scrape the mixture into a shallow 2 1/2quart baking dish (such as a 10 by 10inch
Corning Ware casserole) and pour in the boiling vegetable stock. Cover tightly and bake 60 to 75 minutes, or until all the liquid is absorbed. (Be careful of the steam when removing the cover.) The barley should have a slightly crunchy texture when done. If it is too hard after all the stock has been absorbed, add some boiling stock or water and cook 10 to 15 minutes more.