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