Preheat the oven to 350 ° F.
Cook the pasta as directed on the package. Drain and oil. Reserve.
In a bowl, combine the ricotta, lemon zest, mayonnaise, 1 cup mozzarella cheese and egg. Salt and pepper.
Stir in chicken, red onion, garlic and spinach. Mix well then set aside.
Pour half of the tomato sauce in the bottom of a baking sheet. Stuff the shells with the mixture then place them in the baking sheet. Cover the shells with the rest of the sauce.
Cover with aluminum foil and bake for 30 minutes.
Remove the aluminum foil, cover the shells with the remaining mozzarella cheese and brown in the oven for a few minutes.