Add flour, salt and pepper to a large Oven Bag. Gently squeeze bag to blend ingredients
Add onion, mushrooms, rice and 1 tablespoon butter to oven bag. Turn bag several time to mix ingredients. Arrange ingredients in even layer in bag. Fold down bag opening two times to hold it open; set aside
Microwave water and chicken broth in a medium microwave-safe bowl for about 3 minutes on high power until liquid is very hot. Carefully pour or ladle liquid over ingredients in bag. Carefully unfold bag opening
Close bag with tie and cut air holes
Bake 25 to 30 minutes or until most of the liquid is absorbed. Let stand 5 minutes. Carefully cut top of bag open
Spoon into serving bowl, mix in remaining butter and cheese, then serve