Preheat oven to 375°. Combine spinach, 1-1/4 cup mozzarella cheese, feta cheese and egg in medium bowl; set aside.
Spread 1/2 cup Sauce in 13 x 9-inch baking dish. Layer 4 lasagna noodles, 1/2 of the spinach mixture, 1/2 of the mushrooms and 1/2 cup Sauce. Repeat, starting and ending with noodles. Top with remaining Sauce.
Cover tightly with aluminum foil and bake 25 minutes. Remove foil and sprinkle with remaining mozzarella cheese. Bake uncovered 15 minutes or until cheese is melted. Let stand 10 minutes before serving.