Preheat the oven to 350 degrees . In a large pot of boiling salted water, cook the squash until fork-tender, about 8 minutes; drain.
In a medium bowl, combine the squash, butter, onion, 1/4 cup bread crumbs, the half-and-half and sugar; season with salt and pepper.
Spoon the mixture into a greased 9-by-11-inch baking dish and sprinkle with the remaining 1/4 cup bread crumbs. Bake until golden, 35 to 40 minutes.
