PREHEAT oven to 325ºF. Reserve 2 Tbsp. each onions and tomatoes; set aside. Cook bacon in large skillet until crisp. Remove bacon with slotted spoon to paper towel-covered plate, reserving 1 Tbsp. of the bacon drippings in skillet. Add mushrooms to skillet; cook and stir 2 min. or until tender. Remove from heat. Chop bacon. Add to mushrooms along with the onions and tomatoes; mix well.
BEAT eggs and sour cream with wire whisk until well blended. Pour into greased 13x9-inch baking dish; top with bacon mixture and cheese.
BAKE 30 min. or until centre is set. Sprinkle with reserved onions and tomatoes. Let stand 5 min. before cutting into pieces to serve.