Preheat your oven to 375°F (190°C).
Bring a pot of salted water to a boil and cook the jumbo pasta shells until al dente according to package directions, then drain and set aside.
In a medium skillet, heat the olive oil over medium heat and sauté the ground chicken until fully cooked and browned.
Add the fresh baby spinach to the skillet with the chicken and stir until the leaves are completely wilted, then remove from heat to cool slightly.
In a mixing bowl, combine the ricotta cheese, egg, sea salt, black pepper, and garlic powder, stirring until smooth.
Fold the cooled chicken and spinach mixture into the ricotta mixture until well combined.
Spread half of the marinara sauce onto the bottom of a small oven-safe baking dish.
Stuff each cooked pasta shell generously with the spinach-chicken-ricotta mixture and arrange them in the baking dish.
Spoon the remaining marinara sauce over the shells and sprinkle evenly with the shredded mozzarella cheese.
Cover the dish with foil and bake for 15 minutes, then remove the foil and bake for another 5-10 minutes until the cheese is melted and the sauce is bubbling.
Garnish with freshly chopped basil before serving.