Cut acorn squash in half, lengthwise. Remove and discard seeds and fiber.
In a shallow microwave-safe dish, place squash cut-side down. Put 1 tbsp of water in bottom of dish.
Microwave covered on high power 7 minutes until almost tender.
Meanwhile, combine apple, celery, cranberries, brown sugar, pumpkin pie spice and margarine/butter in a mixing bowl.
When squash is done, turn them over. Fill centers with apple mixture. Microwave covered on high power 5 minutes longer until squash and apple filling are tender. Let stand 3 minutes in microwave after done cooking.