Preheat oven to 375°F (190°C).
Cook large pasta shells in salted boiling water until al dente; drain and cool slightly.
In a bowl, combine ricotta cheese, shredded mozzarella cheese, grated parmesan cheese, beaten egg, chopped fresh parsley, minced garlic, salt, and black pepper.
Stuff each pasta shell generously with the cheese mixture.
Spread a thin layer of marinara sauce on the bottom of a rectangular baking dish.
Arrange stuffed shells in a single layer in the baking dish.
Top shells with remaining marinara sauce and sprinkle additional mozzarella and parmesan cheese on top.
Bake uncovered for 25-30 minutes, until sauce is bubbly and cheese is melted and golden.
Let stand 5 minutes before serving.