Preheat the oven to 375°F
Boil jumbo pasta shells until al dente
Drain the shells and let them cool slightly
Mix ricotta, shredded mozzarella, grated Parmesan, egg, salt, pepper, and chopped parsley in a bowl
Spread a layer of marinara sauce in a baking dish
Fill each shell with the cheese mixture
Place the stuffed shells in the baking dish
Spoon more marinara sauce over the shells
Sprinkle extra mozzarella and Parmesan on top
Cover with foil and bake for 25 minutes
Remove the foil and bake for 10 to 15 minutes more
Let rest for a few minutes before serving