Preheat oven to 375°F (190°C).
Cook pasta shells in salted water until al dente; drain and set aside.
In a bowl, mix ricotta, spinach, mozzarella, Parmesan, egg, garlic, herbs, salt, and pepper.
Spread half of the marinara sauce in a greased baking dish.
Stuff each shell generously with the ricotta-spinach mixture and place in dish.
Pour remaining sauce over shells and drizzle with cream if using.
Sprinkle mozzarella on top.
Cover with foil and bake for 25 minutes.
Uncover and bake an additional 10 minutes until bubbly and lightly golden.