Preheat your oven to 375°F (190°C) and grease a baking dish with olive oil.

Boil the jumbo pasta shells until al dente, about 8 minutes. Drain and let cool slightly.

In a large bowl, combine ricotta cheese, cooked spinach, half of the mozzarella, Parmesan cheese, egg, minced garlic, salt, and pepper. Mix until well blended.

Spread a layer of marinara sauce on the bottom of the baking dish. Fill each pasta shell with the cheese mixture and arrange them in the dish.

Pour the remaining marinara sauce over the stuffed shells and sprinkle the rest of mozzarella cheese on top. Cover with foil and bake for 25 minutes, then uncover and bake for an additional 15 minutes until bubbly and golden brown.

Let cool for a few minutes before serving to help the flavors meld together.