Preheat oven to 350F. Heat the oil in a large skillet. Add mushrooms and onions and cook, stirring until the onions are browned. Stir in the barley, parsley, thyme and pepper(ground). Stir in the cheese; set aside. Rinse the bell peppers. Cut off the tops; remove and discard the seeds and pith. If necessary, make a thin slice on the bottom of each pepper to balance. Spoon 1/4 of the mixture into each pepper. Stand the peppers upright in a baking dish just large enough to accommodate them. Pour the saice into the baking dish. Bake 30 minutes or until the peppers are tender.