1. Prepare stuffing as directed on package. Add 1 egg; mix well. Press onto bottom of 9-inch square baking dish sprayed with cooking spray; sprinkle with 3/4 cup cheese.
2. Cook and stir onions in medium nonstick skillet 5 min. or until crisp-tender. Remove from heat; stir in spinach. Spoon half over cheese layer in baking dish; top with layers of peppers, remaining cheese and remaining spinach mixture. Beat remaining eggs; pour over ingredients in baking dish. Top with tomatoes.