Preheat the oven to 350°F. Bring a large pot of lightly salted water to a boil. Add the manicotti shells and cook until they are tender yet firm, about 8 to 10 minutes.
In a mixing bowl, combine the ricotta cheese, 2 1/2 cups of the mozzarella cheese, 1/4 cup grated Parmesan cheese, egg, Italian seasoning, salt, and pepper.
Spoon the cheese mixture evenly over the pasta. Add the remaining shells on top and pour the rest of the spaghetti sauce evenly over the dish. Sprinkle the remaining 1/2 cup