Bring a large pot of salted water to a boil. Cook the jumbo shells 1–2 minutes less than the package’s listed al dente time, then drain, rinse briefly with cool water, and set aside.

Pat the chicken dry and rub it with 1 tablespoon olive oil, Italian seasoning, garlic powder, 1/4 teaspoon kosher salt, and black pepper. Preheat a grill or grill pan to medium-high, approximately 400–450°F.

Grill the chicken for about 5–7 minutes per side, depending on thickness, until it reaches an internal temperature of 165°F. Rest it for 5 minutes before chopping or shredding.

Heat the oven to 375°F. Stir together the ricotta, egg, Parmesan, 1/2 cup mozzarella, remaining 1/4 teaspoon salt, and chopped grilled chicken.

Spread about 1 cup marinara sauce in a 9-by-13-inch baking dish. Fill each shell with approximately 2 tablespoons of the chicken-cheese mixture and arrange the shells filling side up.

Spoon the remaining marinara over and around the shells, then scatter the remaining 1 cup mozzarella over the top. Cover the dish loosely with foil.

Bake covered for 20 minutes, then remove the foil and bake for another 5–10 minutes, until the sauce bubbles and the cheese is lightly browned. Rest for 5 minutes, garnish with parsley or basil, and serve.