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, 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 to 7 minutes until just cooked through and lightly golden. 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. Gradually whisk in the whole milk and chicken broth. Bring to a gentle simmer and continue cooking for 3 to 4 minutes until the sauce thickens slightly.
05 - Remove the skillet from heat. Whisk the prepared garlic-erb butter 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 cheese, and melted butter. Toss until the mixture is evenly moistened. Sprinkle uniformly over the casserole.
07 - Bake uncovered for 25 minutes, or until the topping is deeply golden and the casserole is bubbling around the edges.
08 - Remove from the oven and let rest for 5 minutes. Garnish with fresh chopped parsley and serve warm.