Preheat your oven to 375°F (190°C). Spread about 1.5 cups of marinara sauce evenly over the bottom of a 9x13 inch baking dish.

Bring a large pot of salted water to a boil. Cook the jumbo shells according to package directions until al dente. Be sure not to overcook them. It's a good idea to cook a few extra shells in case some tear.

Drain the shells in a colander and rinse with cool water to stop the cooking process and make them easier to handle. Set aside.

While the pasta is cooking, prepare the filling. Ensure the thawed spinach is squeezed completely dry to remove all excess moisture. You can use your hands or a cheesecloth for this.

In a large mixing bowl, combine the ricotta cheese, 1 cup of mozzarella cheese, 1/2 cup of Parmesan cheese, the dried spinach, beaten egg, garlic powder, oregano, salt, and pepper. Mix until everything is well combined.

Carefully fill each cooked pasta shell with a generous amount of the cheese mixture (about 2 tablespoons per shell). A small spoon or a piping bag with the tip cut off works perfectly. Arrange the filled shells in a single layer in the prepared baking dish, open-side up.

Spoon the remaining marinara sauce over the top of the stuffed shells.

Sprinkle the remaining 1 cup of mozzarella and 1/4 cup of Parmesan cheese evenly over the sauce.

Cover the baking dish tightly with aluminum foil and bake for 20 minutes.

Remove the foil and continue to bake for another 10-15 minutes, or until the sauce is bubbly and the cheese is melted and lightly golden brown on top.

Remove from the oven and let the stuffed shells rest for 5-10 minutes before serving. This allows them to set up slightly. Garnish with fresh chopped parsley if desired.