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 thoroughly until evenly blended 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 residual drippings and cook for 1 minute, stirring constantly to form a roux. Gradually whisk in whole milk and chicken broth. Bring to a gentle simmer and cook, stirring frequently, until the sauce thickens slightly, about 3–4 minutes.
05 - Remove the skillet from heat and whisk the prepared garlic-erb 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 fresh breadcrumbs, grated Parmesan cheese, and melted butter. Toss until the crumbs are evenly coated. Sprinkle the mixture uniformly over the sauced chicken.
07 - Bake uncovered on the center rack for 25 minutes, or until the topping is deeply golden and the casserole is bubbling at the edges.
08 - Remove from the oven and allow the casserole to rest for 5 minutes. Garnish with freshly chopped parsley and serve warm.