Add mushrooms; cook 4 min. or until tender, stirring occasionally. Beat eggs, milk and mayo in large bowl with whisk until blended. Stir in stuffing mix.
Spoon into 9-inch square baking dish sprayed with cooking spray; top with mushrooms, tomatoes and onions.
Bake 35 min.
Sprinkle with cheese; bake 15 min. or until center is set and cheese is melted.