Cook the jumbo pasta shells according to package directions until al dente. Drain and rinse with cold water to prevent sticking.
In a medium bowl, combine the ricotta cheese, mozzarella cheese, Parmesan cheese, egg, chopped parsley, salt, and pepper. Mix well until thoroughly combined.
Spread about half of the marinara sauce in the bottom of a 9x13 inch baking dish.
Carefully stuff each cooked pasta shell with the ricotta cheese mixture. Arrange the stuffed shells in a single layer over the marinara sauce in the baking dish.
Pour the remaining marinara sauce over the stuffed shells, ensuring they are mostly covered.
Cover the baking dish with aluminum foil and bake for 20 minutes.
Remove the foil and bake for another 10-15 minutes, or until the sauce is bubbly and the cheese is melted and slightly golden.
Let stand for 5 minutes before serving.
Notes
Garnish with extra fresh parsley or a sprinkle of Parmesan cheese before serving, if desired.