Preheat oven to 350 degrees. Coat a 9 x 12 baking dish with cooking spray.
Cook lasagna noodles according to package directions. Once cooked, rinse and drain. Lay noodles on paper towels and blot them with paper towels to completely dry.
In the meantime, on a microwave-safe plate or small bowl, add onions and cook in the microwave for about 2 minutes, until soft.
Combine cooked onions, spinach, ricotta, Parmesan, egg, and a little salt and pepper in a medium bowl. Ladle about 1 cup sauce on the bottom of the baking dish.
Take ⅓ cup of ricotta mixture and spread evenly over each noodle. Roll carefully and place seam side down onto the baking dish. Repeat with remaining noodles.
Ladle sauce over the noodles in the baking dish and top each evenly with the mozzarella cheese. Cover the baking dish with foil and bake for 35 minutes, or until cheese is melted.
To serve: spoon a little sauce on the plate and top with 1 lasagna roll-up.