Cook pasta shells until al dente, drain, and cool on a tray with a drizzle of olive oil.
Mix chicken, spinach, ricotta, ½ cup mozzarella, parmesan, egg, garlic, Italian seasoning, salt, and pepper until combined.
Spread 1 cup marinara sauce in the bottom of a 9x13 baking dish.
Fill each pasta shell with about 2 tablespoons of the mixture and place into the dish.
Top with remaining marinara sauce and sprinkle with the remaining 1 cup mozzarella cheese.
Cover with foil and bake at 375°F (190°C) for 25 minutes, then uncover and bake 10–15 minutes more until golden and bubbly.
Let rest for 5 minutes, garnish with fresh basil, and serve hot.