01Preheat the oven to 375 degrees F. Coat a 9 x 13-inch baking dish with cooking spray. Spread half of the marinara sauce on the bottom of the prepared dish.
02Cook pasta shells according to package directions to al dente (about 9 minutes). Drain and spread on a baking sheet to cool.
03While the pasta cooks, in a small microwave-safe bowl, microwave frozen spinach on high for 2 minutes, stirring halfway through. Remove from the microwave and set aside to cool.
04In a large microwave-safe bowl, microwave frozen cauliflower rice on high for3 minutes. Using an immersion blender or food processor, puree cauliflower rice until smooth. Add the thawed spinach, ricotta, 1 cup mozzarella, 4 tablespoons Parmesan, egg, parsley, garlic, oregano, salt, and pepper and mix until combined.
05Using a spoon, fill cooled shells with spinach cheese mixture. Place shells seam side up in the baking dish. Cover with remaining sauce. Top with the remaining mozzarella and remaining 2 tablespoons of Parmesan.
06Cover the dish with foil and bake for 30 minutes. Remove foil and bake for another 10 minutes. Turn the oven setting to broil.
07Broil for 1 to 2 minutes right before serving for a more golden top. Let cool for approximately 5 minutes before serving.