Preheat oven to 350°F. Lightly grease a shallow baking or casserole dish.
Cut each squash in half lengthwise. Remove and discard seeds. Place squash, cut side down, in prepared baking dish. Bake for 30 minutes.
Meanwhile, saute onion and butter in large skillet for 3 minutes. Add sausage and cook until no longer pink. Remove from heat and add other ingredients except brown sugar. Shape mixture into 4 equal balls.
Remove squash from oven. Turn squash over. Sprinkle with brown sugar. Place sausage ball in each squash cavity. Cover the baking dish and return to oven. Bake 15 minutes, then uncover and bake 15 minutes more until squash is tender.