Preheat oven to 375°F (190°C).
Cook jumbo pasta shells according to package instructions until al dente. Drain and set aside.
In a skillet, heat olive oil over medium heat. Add minced garlic and sauté until fragrant.
Add fresh spinach to the skillet and cook until wilted. Remove from heat and let cool.
In a mixing bowl, combine ricotta cheese, half of the shredded mozzarella, grated parmesan, beaten egg, cooked spinach, salt, and black pepper. Mix until well combined.
Stuff each cooked pasta shell with the cheese and spinach mixture.
Spread a thin layer of marinara sauce on the bottom of a white rectangular baking dish.
Arrange stuffed shells in a single layer on top of the sauce.
Pour remaining marinara sauce over the shells.
Sprinkle the remaining shredded mozzarella cheese evenly over the top.
Bake in the preheated oven for 30-35 minutes, until the cheese is melted and bubbly with golden brown edges.
Remove from oven and garnish with fresh basil leaves before serving.