Heat oven to 375°F. Spread 2 cups marinara in a 9x13-inch baking dish.
Boil shells in salted water until barely al dente, 1–2 minutes shy. Drain and rinse.
Toss shells with 1 teaspoon olive oil so they don’t stick. Set aside to cool slightly.
Combine chicken, ricotta, 1½ cups mozzarella, ½ cup Parmesan, egg, parsley, garlic, seasoning, salt, and pepper.
Stir in red pepper flakes if using. Mix until evenly combined and scoopable.
Make topping: mix panko, remaining ¼ cup Parmesan, and 1 tablespoon olive oil with a pinch of salt.
Toast topping in a skillet over medium heat 2–3 minutes, stirring, until lightly golden. Cool briefly.
Fill each shell with 2–3 tablespoons of the chicken mixture.
Nestle stuffed shells over the sauce. Spoon remaining 1 cup marinara across the tops.
Sprinkle remaining ½ cup mozzarella over shells, then scatter the toasted breadcrumb topping.
Cover with foil and bake 20 minutes. Uncover and bake 10–15 minutes, until bubbling and browned.
Rest 5 minutes before serving. Garnish with extra parsley and serve hot.