Beat eggs and sour cream with whisk until well blended. Pour into greased 13x9-inch baking dish. Bake 10 minutes or until egg mixture is softly set. Meanwhile, cook sausage, mushrooms, and onions in large skillet on medium heat 6 to 8 minutes or until sausage is done, stirring occasionally. Drain.

Reduce oven temperature to 325 degrees Fahrenheit. Spoon tomatoes over egg layer; cover with sausage mixture and cheese.

Bake 30 minutes or until center is set.