01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish and set aside.
02 - In a small bowl, combine softened butter, minced garlic, chopped parsley, dill (if using), salt, pepper, and lemon juice. Mix thoroughly until evenly blended and set aside.
03 - Heat olive oil in a large skillet over medium-high heat. Add chicken pieces, season lightly with salt and pepper, and sauté until just cooked through and lightly golden, about 5 to 7 minutes. Transfer chicken to the prepared baking dish.
04 - In the same skillet, add flour and cook for 1 minute, stirring constantly to form a roux. Gradually whisk in milk and chicken broth. Bring to a simmer and cook until slightly thickened, about 3 to 4 minutes.
05 - Remove skillet from heat and whisk in the prepared garlic-herb butter until smooth and emulsified. Pour the sauce evenly over the chicken in the baking dish.
06 - In a medium bowl, combine breadcrumbs, 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 golden brown and the casserole is bubbling throughout.
08 - Remove from the oven and let rest for 5 minutes. Garnish with fresh parsley before serving.