1Brown ground beef in skillet, drain. Stir in mushrooms and 1/2 can onions. Spoon into 13x9 baking dish.
Bring soup to a boil in saucepan, remove from heat & stir in 1/2 cup sour cream. Spoon over ground beef mixture.
2Cut biscuits into halves. Arrange over top of ground beef mixture; sprinkle with remaining onions. Combine 1/2 cup sour cream, egg, celery seed and salt in bowl; mix well. Spoon over casserole.
Bake at 375^ for 25 to 30 minutes or until biscuits are golden brown.