HEAT oven to 400°F.
BEAT eggs and sour cream with whisk until well blended. Pour into 13x9-inch baking dish sprayed with cooking spray. Bake 10 min. or until egg mixture is softly set. Meanwhile, cook sausage, mushrooms and onions in large skillet on medium heat 6 to 8 min. or until sausage is done, stirring occasionally. Drain.
REDUCE oven temperature to 325°F. Spoon tomatoes over egg layer; cover with sausage mixture and cheese.
BAKE 30 min. or until center is set.