Halve squash lengthwise; discard seeds. Place squash in a microwave-safe dish, cut side down. Microwave, covered, on high until tender, 10-12 minutes. , Meanwhile, in a large skillet, cook and crumble sausage with onion over medium heat until no longer pink, 5-7 minutes; drain. Remove from heat; stir in spinach, cranberries and bread crumbs. In a small bowl, whisk together egg and milk; add to sausage mixture and toss until moistened. , Turn over squash; fill with sausage mixture. Microwave, covered, until a thermometer inserted in stuffing reads 165°, 2-3 minutes.