HEAT oven to 400ºF.
PREPARE stuffing as directed on package. Add 1 egg; mix well. Press onto bottom of greased 9-inch square baking dish; sprinkle with 3/4 cup cheese.
COOK and stir onions in medium nonstick skillet sprayed with cooking spray 5 min. or until tender. Remove from heat; stir in spinach. Spoon half over cheese layer in baking dish; top with peppers. Cover with layers of remaining cheese and spinach mixture. Beat remaining eggs; pour over ingredients in baking dish. Top with tomatoes.
BAKE 35 to 40 min. or until center is set.
