Boil pasta shells in salted water until just al dente, then drain and cool
In a bowl, combine ricotta, sun-dried tomatoes, Parmesan, half of mozzarella, garlic, egg, Italian seasoning, salt, pepper, and red pepper flakes
Mix until creamy and well combined
Preheat oven to 375°F (190°C) and lightly grease baking dish
Spread a layer of marinara sauce on the bottom of the dish
Fill each shell with about 2 tablespoons of the ricotta mixture
Arrange stuffed shells in a single layer in the dish
Pour remaining marinara sauce over the shells
Sprinkle remaining mozzarella and extra Parmesan on top
Cover with foil and bake for 20 minutes
Remove foil and bake another 10 minutes until cheese is bubbly and golden
Let rest for 5 minutes before serving and garnish with fresh herbs