Preheat oven to 350°F. Coat 13 x 9 x 2-inch baking dish with nonstick cooking spray.
In medium bowl, stir together ricotta, Parmesan, and egg.
Spread 1 cup pasta sauce over bottom of baking dish. Arrange 3 lasagna noodles side by side in dish. Spread 1 1/4 cups ricotta mixture over top of noodles. Top with a third of the spinach. Repeat layering two more times with sauce, noodles, ricotta mixture, and spinach. Top with remaining 3 noodles. Spread remaining 1 cup sauce over top. Gently press lasagna noodles down into dish, so sauce comes up around sides. Cover dish with foil.
Bake 35 minutes. Uncover. Sprinkle with mozzarella. Bake until cheese is melted and filling is bubbly, about 10 minutes. Let stand 10 minutes before cutting into rectangles.