Preheat the oven to 375°F (190°C) and lightly spray an 8×8-inch baking dish with nonstick spray; set aside.
Bring a large pot of salted water to a boil and cook the jumbo shells until al dente according to package directions. Drain and arrange the shells upside down on a baking sheet to dry and cool slightly.
Heat the olive oil in a large skillet over medium heat. Add the minced garlic and cook until fragrant, about 1–2 minutes, taking care not to brown it.
Add the baby spinach to the skillet and cook, stirring occasionally, until wilted, about 3–4 minutes. Remove from heat and let cool slightly, then squeeze out excess moisture if needed and chop coarsely.
In a large mixing bowl, combine the cooked spinach, ricotta, shredded mozzarella, grated parmesan, egg, salt, and black pepper until well mixed.
Spread half of the marinara sauce in the bottom of the prepared baking dish.
Working one shell at a time, spoon a generous amount of the ricotta–spinach mixture into each shell and place it filling-side up in the baking dish.
When all shells are filled and arranged, spoon the remaining marinara sauce over the shells and sprinkle additional shredded mozzarella on top.
Cover the dish tightly with aluminum foil and bake for 20 minutes.
Remove the foil and bake for another 15 minutes, until the cheese is bubbly and lightly browned.
Serve hot with a sprinkle of grated parmesan and fresh basil leaves.
