01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with butter or non-stick spray.
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 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 5–7 minutes, turning occasionally, until the chicken is 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 whole milk and chicken broth while whisking continuously to prevent lumps. Bring the mixture to a gentle simmer and cook for 3–4 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 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 breadcrumbs, grated Parmesan cheese, and melted butter. Toss until the breadcrumbs 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 freshly chopped parsley before serving.