Stir together the first 5 ingredients in a large bowl.
Place the spinach in a dish towel or a triple layer of paper towels and squeeze the water out.Add spinach to the mayonnaise mixture and stir in. Sir in the chicken, cheddar cheese, and pepper.Check for seasoning and add salt to taste.
Transfer mixture to a greased 11x7 or 13x9-inch baking dish.
Sprinkle the Parmesan cheese on top.
Sprinkle the bread crumbs on top and then drizzle with the butter.
Bake at 350 degrees uncovered for 30 minutes.