This barley and mushroom casserole is a tasty side dish, and it goes particularly well with grilled steaks or roast beef. The mushrooms give the barley earthy flavor. It's made with the addition of chicken stock, but vegetable broth or mushroom broth may be used for a vegetarian dish. Use plain button mushrooms, portobello, or a wild mushroom assortment. For serving, add a grating of parmesan cheese, or top it with some shredded mozzarella cheese, fontina, or queso blanco just before the casserole comes out of the oven.