Preheat oven to 350. Grease a 13 x 9 baking dish.
Place a large pot full of salted water over high heat and bring to a boil. Cook the jumbo shells until al dente according to package directions, about 9 minutes. Drain, rinse with cold water, and set aside.
While the pasta is cooking, make the filling. To a mixing bowl, add the spinach, artichokes, lemon zest and juice, parmesan, 1 cup of the mozzarella, and ricotta cheese. Add in the egg, garlic powder, salt, and a generous pinch of black pepper. Fold the ingredients together until well combined. Scoop the filling into a pastry bag with a large opening.
Spoon 1 cup of marinara sauce int the bottom of the prepared baking dish, spreading evenly to coat. When the pasta is ready, fill each shell with the spinach and artichoke filling and place the shells into rows in the marinara sauce, cheese side up. When all the shells are filled, spoon the remaining marinara over top of the shells. Sprinkle with remaining mozzarella cheese. Cover the dish with foil and bake 30 minutes. Remove the foil, turn the oven up to broil, and cook another 5-7 minutes, or until the cheese and shells begin to brown on the edges. Serve.