Preheat oven to 375°F (190°C).
Cook pasta shells in salted water until al dente, then drain and set aside.
Heat olive oil in a skillet and sauté garlic until fragrant. Add chopped spinach and cook until wilted.
In a bowl, combine ricotta, Parmesan, half the mozzarella, spinach mixture, egg, salt, and pepper.
Spread 1/2 cup marinara sauce over the bottom of a baking dish.
Stuff each shell with the ricotta-spinach filling and arrange in the dish.
Pour remaining marinara over the shells and sprinkle with the rest of the mozzarella.
Cover with foil and bake for 20 minutes. Remove foil and bake another 10 minutes until cheese is bubbly.
Garnish with fresh basil before serving.