Preheat the oven to 350 degrees. Melt the butter and combine it with the breadcrumbs in a large bowl. Put one-half of the breadcrumb mixture in the bottom of a baking dish sprayed with non-stick spray. Combine the squash, onion, pimentos, chicken soup, and sour cream in a large bowl. Mix well. Pour into the casserole dish on top of the breadcrumbs. Top the casserole with the remaining breadcrumbs and for 45 minutes. Remove from oven, either serve immediately or let cool to room temperature, cover and refrigerate.