Cook the lasagna noodles in a large pot of salted water. Aim for “al dente” texture, which usually means boiling for two minutes less than the box instructions. Drain them and lay them flat and separate on parchment paper to prevent sticking.
Take your thawed spinach and squeeze it inside a clean kitchen towel until it is bone-dry.
In a medium bowl, mix the ricotta, dry spinach, egg, Parmesan, and half of the mozzarella. Stir in the garlic powder, oregano, salt, and pepper.
Spread a thin layer of marinara sauce on the bottom of your baking dish. Lay a noodle flat and spread two tablespoons of the cheese mixture along its length. Roll the noodle up tightly and place it seam-side down in the pan.
Pour the remaining marinara sauce over the rolls, making sure to coat the exposed edges of the pasta. Sprinkle the rest of the mozzarella over the sauce.
Cover the dish with foil and bake at 375°F (190°C) for 20 minutes. Remove the foil and bake for another 10 to 15 minutes until the cheese is bubbling and golden. Let the rolls rest for five minutes before serving.