Bring a large pot of well-salted water to a rolling boil. Cook the jumbo pasta shells until al dente according to the package instructions. Drain and set aside to cool slightly.
In a large skillet over medium-high heat, warm the olive oil. Add the ground chicken or chicken pieces, and season with salt and Italian seasoning. Cook, stirring occasionally, for 5 to 6 minutes until the chicken is no longer pink and cooked through. Transfer the chicken to a plate and allow it to cool to room temperature.
Preheat your oven to 350°F (175°C).
In a mixing bowl, combine the cooled chicken with ricotta, 1 cup of mozzarella, Parmesan cheese, egg, garlic, and salt. Stir until fully incorporated and creamy.
Spread 2 cups of marinara sauce evenly across the bottom of a 9x13-inch baking dish.
Fill each cooked pasta shell with about 2 tablespoons of the chicken and cheese mixture. Arrange the filled shells in the baking dish, open side facing up.
Spoon the remaining marinara sauce over the top of the shells. Sprinkle with the remaining 1 cup of mozzarella cheese and finish with a layer of Panko breadcrumbs for added crunch.
Cover the baking dish with foil—spraying the underside of the foil with nonstick spray to prevent sticking. Bake for 20 minutes, then uncover and bake for another 5 to 10 minutes until the Panko is golden and crisp.
Garnish with chopped parsley before serving, if desired.