Preheat the oven to 375°F (191°C). Lightly grease a 9×13-inch baking dish or similar size (about 3-quart) casserole dish. Set aside.

Bring a large pot of salted water to a boil and add the jumbo shells. Boil the shells until just al dente, about 9 minutes. Drain and rinse with cold water. Lay the shells, open side up, on a baking sheet.

Heat olive oil in a large skillet over medium heat. Sauté the garlic for 30 seconds, then add the spinach. Cook, stirring, until the spinach wilts and cooks down, about 3 minutes. Remove from heat.

In a large bowl, mix together the ricotta cheese, salt, pepper, Italian seasoning, egg, parmesan, and 1 cup (113g) of the mozzarella. Fold in the chicken and garlic-spinach mixture.

Pour about 2 cups (about 450g) of sauce into the prepared baking dish to just cover the bottom.

Spoon the filling evenly into the shells and arrange in the baking dish.

Pour the remaining sauce over the stuffed shells. Sprinkle with the remaining mozzarella cheese.

Bake, uncovered, for 20–25 minutes. Serve with fresh basil and extra parmesan, if desired.