Cut the crookneck squash in half lengthwise. Place them in a shallow ceramic or tempered glass baking dish, cover tightly with microwave-safe plastic wrap and cook on High for 3 to 4 minutes, until fork tender.
While the squash is cooking, coarsely chop the onion and the cashews. Melt the butter in a medium frying pan. Add the onion and cook over medium heat, stirring occasionally, until the onion is translucent, 2 to 3 minutes. Preheat the broiler.
Remove the squash from the microwave. With a spoon, scoop out some of the centers, being careful to leave a 1/8-inch thick shell. Chop the removed squash and stir into the onion along with the cashews and 1/4 cup of the bread crumbs.
Fill the squash shells with the stuffing mixture. Sprinkle the remaining bread crumbs on top. Place the stuffed squash on a broiler pan and broil about 4 inches from the heat, until golden brown on top, 2 to 3 minutes.