Mix egg, ricotta and Parmesan in medium bowl until blended. Stir in 1 cup mozzarella.
Reserve 9 pepperoni slices, and 2 Tbsp. each peppers and onions. Quarter remaining pepperoni slices; place in separate medium bowl. Add pasta sauce, water, remaining peppers and onions; mix well.
Spread 1/2 cup sauce onto bottom of 8-inch square baking dish sprayed with cooking spray. Top with layers of 2 lasagna noodles, broken to fit; 1/3 of the ricotta mixture; and 1/2 cup of the remaining sauce. Repeat layers twice. Top with remaining noodles, sauce, mozzarella, and reserved pepperoni, peppers and onions. Cover with foil sprayed with cooking spray.
Bake 45 min. or until heated through, uncovering for the last 10 min. Let stand 10 min. before cutting to serve.