Preheat oven to 350°F. Grease a 9x13 inch baking dish.
In a large bowl, combine the chicken, bread crumbs, Parmesan cheese, parsley, garlic powder, onion powder, oregano, basil, pepper, and salt. Stir in the ricotta and mozzarella cheeses. Mix until well combined.
Cook the pasta shells according to package directions. Drain and rinse with cold water.
Spread 1/2 cup of marinara sauce in the bottom of the prepared baking dish. Stuff each cooked shell with the chicken mixture and arrange in the dish. Top with the remaining 2 1/2 cups of marinara sauce.
In a medium saucepan, melt the butter over medium heat. Add the flour and stir until smooth. Gradually stir in the milk and bring to a boil. Reduce heat and simmer until thickened, about 5 minutes. Stir in the garlic powder, onion powder, salt, and pepper. Pour the sauce over the stuffed shells.
Sprinkle the top with the mozzarella cheese. Bake for 30 minutes, or until cheese is melted and bubbly.
Interesting Facts
The dish is believed to have originated in the city of Naples, Italy in the 19th century.
Stuffed shells are often referred to as 'conchiglie ripiene' in Italy, which translates to 'stuffed shells'.
This dish is popular in Italian-American cuisine.
The shells can also be filled with a variety of ingredients such as ricotta and spinach, ground beef, or sausage.