Wild Rice, Barley and Mushroom Casserole

1/2 cup unsalted butter
1 medium onion, chopped
8 oz. wild rice
2 Tbsp. pearl barley
28 oz. canned beef broth
1/4 tsp. dried thyme
8 oz. sliced mushrooms

Melt 1/4 cup butter in 3?quart microwave casserole on high. Add onion and cook uncovered on high until soft, about 1 minute. Add rice, barley, beef broth, and thyme. Cover tightly and cook on high 25 minutes. Reduce to medium, stir, and cook until most liquid is absorbed, about 40 minutes. Let stand, covered, for about 10 minutes.

Melt remaining 1/4 cup butter in shallow 10?inch microwave dish on high. Add mushrooms. Cook uncovered on high until mushrooms are tender, stirring twice, 4 to 5 minutes. Stir mushrooms into cooked rice and barley mixture. Serves 6.