1Preheat oven to 350 degrees. Melt butter in a large skillet over medium high heat.
2Add the onions, carrot, celery, mushrooms, barley, thyme (if using it) and saute, stirring frequently, until the mushrooms begin to brown and the barley starts to toast, about 10 minutes. Add salt and pepper after mushrooms have browned.
3Place mixture into a shallow 2 1/2 quart baking dish (like a 10 by 10 Corning Ware casserole) and pour in the hot stock. Cover tightly and bake 60 to 75 minutes, or until all the liquid is absorbed. Be careful of the steam when removing the top.
4The barley should have a slightly crunchy texture when done. If it’s too hard after all the stock has been absorbed, add some boiling stock or water and cook another 10 to 15 minutes.