HEAT oven to 325 degrees F. Reserve 2 Tbsp. each onions and tomatoes. Cook bacon in large skillet until crisp. Remove bacon from skillet, reserving 1 Tbsp. drippings in skillet. Drain bacon on paper towels. Add mushrooms to reserved drippings; cook and stir 2 min. or until tender. Remove from heat. Crumble bacon. Add to skillet with remaining onions and tomatoes; mix well.
BEAT eggs and sour cream with whisk until well blended. Pour into greased 13x9-inch baking dish; top with bacon mixture and cheeses.
BAKE 30 min. or until center is set. Top with reserved onions and tomatoes. Let stand 5 min. before cutting to serve.
Crustless Garden Vegetable Quiche Prepare as directed, stirring in 1 thinly sliced zucchini with the mushrooms.