Bring a large pot of salted water to a boil. Add the jumbo pasta shells and cook according to package instructions until al dente, usually about 9-11 minutes. Stir occasionally to prevent sticking. Drain and rinse with cold water to stop the cooking process. Set aside on a baking sheet lined with parchment paper, ensuring they don’t stick together.
Heat 2 tablespoons of olive oil in a large skillet over medium heat. Add the ground chicken or finely chopped chicken breasts, 1 teaspoon dried Italian seasonings, and 1/2 teaspoon salt. Cook, stirring occasionally, until the chicken is no longer pink and cooked through, about 6-8 minutes. Add the minced garlic during the last minute of cooking to release its flavor without burning. Remove from heat and let cool slightly.
In a large mixing bowl, combine 2 cups of ricotta cheese, 1 cup of shredded mozzarella, 1/2 cup grated Parmesan, 1 large egg, and the cooked chicken mixture. Mix well until all ingredients are evenly incorporated. This mixture should be creamy and slightly firm enough to hold its shape when stuffed.
Preheat your oven to 375°F (190°C). Spread 2 cups of marinara sauce evenly on the bottom of your baking dish. Carefully spoon the chicken and cheese filling into each jumbo shell, filling them generously but not overstuffing. Place each stuffed shell in the baking dish over the marinara sauce.
Pour the remaining 2 cups of marinara sauce evenly over the stuffed shells. Sprinkle the remaining 1 cup of mozzarella cheese and 1/2 cup Panko breadcrumbs over the top for a cheesy, crispy finish. Cover the baking dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 10 minutes or until the topping is golden and bubbly.
Remove from the oven and let the dish rest for 5 minutes before serving. Sprinkle chopped parsley on top for a fresh pop of color. Serve alongside a crisp salad or crusty bread.