Spread 1 cup of 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.