01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish and set aside.
02 - In a small bowl, combine the softened butter, minced garlic, chopped parsley, dill (if using), salt, black pepper, and lemon juice. Mix thoroughly until evenly blended and set aside.
03 - Heat olive oil in a large skillet over medium-high heat. Season the chicken pieces lightly with salt and pepper, then sauté until just cooked through and lightly golden, approximately 5 to 7 minutes. Transfer the seared chicken to the prepared baking dish in an even layer.
04 - In the same skillet, sprinkle the flour over the remaining drippings and cook for 1 minute, stirring constantly. Gradually whisk in the whole milk and chicken broth. Bring the mixture to a simmer and cook until the sauce thickens slightly, about 3 to 4 minutes.
05 - Remove the skillet from heat and whisk the prepared garlic-herb butter into the sauce until smooth and emulsified. Pour the sauce evenly over the chicken in the baking dish.
06 - In a medium bowl, combine the panko breadcrumbs, grated Parmesan cheese, and melted butter. Toss until the mixture is evenly coated. Sprinkle the topping uniformly over the casserole.
07 - Bake the casserole uncovered for 25 minutes, or until the topping is golden brown and the sauce is bubbling around the edges.
08 - Remove from the oven and let the casserole rest for 5 minutes. Garnish with fresh chopped parsley before serving.