Preheat an oven to 375 degrees F (190 degrees C). Grease a 2-quart baking dish.
Place the squash in a large saucepan, cover with water, and boil until soft, about 15 minutes. Drain the squash well, place in a large mixing bowl, and mash until slightly chunky. Stir in 1/2 cup of bread crumbs, onion, eggs, 1/4 cup of butter, sugar, salt, and pepper until thoroughly combined, and spread mixture into the prepared baking dish. Drizzle the top of the casserole with 1/4 cup melted butter, and sprinkle 1/4 cup bread crumbs over the butter.
Bake in the preheated oven until the casserole is cooked through and the top is golden brown, about 1 hour.