01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with butter or nonstick spray.
02 - In a small bowl, combine the softened butter, minced garlic, chopped parsley, dill (if using), salt, pepper, and lemon juice. Mix thoroughly until all ingredients are evenly incorporated. Set aside.
03 - Heat olive oil in a large skillet over medium-high heat. Season the chicken pieces lightly with salt and pepper, then add to the skillet. Sauté for 5 to 7 minutes, turning occasionally, until the chicken is just cooked through and lightly golden on the edges. Transfer the seared chicken to the prepared baking dish in an even layer.
04 - In the same skillet, sprinkle the flour over the residual drippings and cook for 1 minute, stirring constantly to form a roux. Gradually pour in the milk and chicken broth while whisking continuously to prevent lumps. Bring the mixture to a gentle simmer and cook for 3 to 4 minutes until the sauce thickens slightly and coats the back of a spoon.
05 - Remove the skillet from heat. Add the prepared garlic-erb butter compound to the sauce and whisk until completely melted and smooth. Pour the finished sauce evenly over the chicken in the baking dish.
06 - In a medium bowl, combine the panko breadcrumbs, grated Parmesan, and melted butter. Toss until the crumbs are evenly coated. Sprinkle the mixture uniformly over the casserole.
07 - Bake uncovered on the center rack for 25 minutes, or until the topping is deep golden brown and the sauce is bubbling around the edges.
08 - Remove the casserole from the oven and let it rest for 5 minutes to allow the sauce to settle. Garnish with fresh chopped parsley before serving.