Preheat oven to 350. Line the bottom of a buttered 13x9 baking pan with the bread. Cut the bread slices to fit. Spread the chicken out evenly over top of the bread.
In a small frying pan, melt 2 tablespoons of butter over medium heat. Add mushrooms and saute until slightly browned, about 3-5 minutes. Remove from heat, add mayonnaise. Mix well and spoon evenly over chicken.
Sprinkle cheese evenly over mushroom mixture.
In a large bowl, combine milk, soup, eggs, pimiento, salt and pepper. Mix well and pour evenly over top.
Bake 1 hour until toothpick inserted near center comes out clean.
Sprinkle breadcrumbs over top and dot with slivers of the remaining 2 tablespoons of butter.
Bake for about 10 minutes more or until the breadcrumbs are lightly browned.