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, pepper, and lemon juice. Mix thoroughly until all ingredients are evenly incorporated 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 to 7 minutes. Transfer the seared chicken to the prepared baking dish in an even layer.
04 - In the same skillet, sprinkle flour over the residual drippings and cook for 1 minute, stirring constantly to form a roux. Gradually whisk in the milk and chicken broth, ensuring no lumps remain. Bring to a simmer and cook until the sauce coats the back of a spoon, about 3 to 4 minutes.
05 - Remove the skillet from heat and whisk the prepared garlic-herb butter compound into the sauce until completely smooth and emulsified. 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 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 to allow the sauce to settle. Garnish with fresh chopped parsley before serving.