Start by boiling a large pot of salted water. Once it reaches a rolling boil, carefully add the jumbo pasta shells. Cook according to package instructions until al dente, usually about 8-10 minutes. Drain the shells and rinse them under cold water to stop the cooking process. Set them aside on a clean kitchen towel to dry.
In a large mixing bowl, combine the shredded chicken, chopped spinach, ricotta cheese, half of the mozzarella cheese, grated Parmesan cheese, minced garlic, Italian seasoning, egg, salt, and pepper. Use a wooden spoon to mix everything together until well combined.
Preheat your oven to 375°F (190°C). Grease your baking dish with a drizzle of olive oil. Take a cooked shell and carefully spoon a generous amount of the chicken and spinach filling into each shell. Place the stuffed shells seam side up in the prepared baking dish. Repeat until all shells are filled.
Pour the marinara sauce evenly over the stuffed shells, making sure each shell is coated. Sprinkle the remaining mozzarella cheese on top for that gooey, cheesy finish.
Cover the baking dish with aluminum foil and bake in the preheated oven for about 25 minutes. After 25 minutes, remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden brown.