01 - Preheat the oven to 375°F. Lightly grease a 9x13-inch baking dish with nonstick cooking spray or oil.
02 - In a mixing bowl, combine the softened butter, minced garlic, parsley, dill (if using), lemon juice, salt, and pepper. Blend until the mixture is smooth and fully incorporated. Set aside.
03 - Distribute the chicken pieces evenly across the bottom of the prepared baking dish in a single layer.
04 - Dot spoonfuls of the garlic-herb butter mixture over the arranged chicken pieces, spacing them evenly.
05 - Pour the heavy cream and chicken broth evenly over the chicken. Sprinkle the grated Parmesan cheese across the top.
06 - If desired, sprinkle shredded mozzarella evenly over the casserole for an extra cheesy layer.
07 - In a small bowl, toss the panko breadcrumbs with melted butter and dried thyme or parsley until evenly coated. Sprinkle the mixture uniformly over the casserole surface.
08 - Bake uncovered for 30 to 35 minutes, until the breadcrumb topping is golden brown and the chicken is fully cooked through (internal temperature reaches 165°F).
09 - Remove from the oven and let the casserole rest for 5 minutes to allow the sauce to set before serving.