Bring a large pot of salted water to a boil and cook the jumbo shells according to package directions until al dente; drain and spread shells individually on a cutting board or baking sheet so they don't stick.
Preheat the oven to 350°F (175°C) and lightly grease two 9x13-inch baking dishes (or one if preferred).
In a large bowl, combine the chopped cooked chicken, chopped spinach, chopped onion, minced garlic, ricotta, softened cream cheese, shredded Parmesan, beaten egg, dried parsley, Italian seasoning, salt, and pepper; mix until evenly combined.
Stir 1 cup of the shredded mozzarella into the filling mixture.
Spoon or pipe the filling into each shell using a spoon or a zip-top bag with the corner snipped; fill all shells evenly.
Spread half of the spaghetti sauce across the bottom of the prepared baking dishes, arrange the filled shells over the sauce, then spoon the remaining sauce over the shells; cover tightly with foil.
Bake covered for 40 minutes. Remove the foil, sprinkle with the remaining 1 cup mozzarella, then bake uncovered 5 to 10 more minutes until cheese is melted and bubbly.
Let the dishes rest a few minutes before serving.
