HEAT oven to 400°F.

PREPARE stuffing mix 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.

COOK and stir onions in medium nonstick skillet 5 min. or until 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.

BAKE 40 to 45 min. or until centre is set.