Preheat oven to 200°C. Cook broccoli in a saucepan of boiling water for 2 minutes or until just tender. Drain.
Remove meat from chicken and roughly chop. Discard skin and bones.
Heat cream cheese, milk and mustard in a saucepan over medium heat, stirring until smooth. Remove from heat and add 1/2 cup cheese, stirring until melted and combined. Squeeze liquid from spinach. Combine chicken, broccoli, sauce and spinach in an 8-cup capacity ovenproof dish.
Combine breadcrumbs, butter and remaining cheese in a small bowl. Sprinkle over chicken mixture. Bake for 20 minutes or until the crumb topping is golden and the bake is heated through. Serve with salad if desired.