If you don't have cooked chicken, place the 4 chicken breast halves in a large pot, cover with water, and simmer for 20–30 minutes until cooked through; drain and shred or slice into bite-sized pieces.
Preheat the oven to 350°F (175°C) and spray a 9x9-inch baking dish with cooking spray.
Place the cooked broccoli florets in the bottom of the prepared baking dish and arrange the cooked chicken pieces on top of the broccoli.
In a mixing bowl, combine the cream of chicken soup, milk, mayonnaise, salad dressing, grated cheddar cheese, and lemon juice; stir until smooth.
Spread the soup-and-cheese mixture evenly over the chicken and broccoli in the baking dish.
Sprinkle the bread crumbs and grated Parmesan evenly over the top, then dot with the 2 tablespoons of butter.
Bake at 350°F (175°C) for 30 minutes, until the topping is golden and the casserole is heated through.