1Preheat oven to 400 degrees F (200 degrees C). Lightly grease a baking sheet with olive oil.
2Place squash, cut-side down, on baking sheet.
3Combine apple, celery, water, onion, and butter in a small baking dish; cover with aluminum foil.
4Bake squash and apple mixture in the preheated oven until squash and apple are tender, about 45 minutes. Fill squash halves with apple mixture; drizzle with maple syrup and sprinkle with cinnamon.
Footnotes
Tip
Aluminum foil helps keep food moist, ensures it cooks evenly, keeps leftovers fresh, and makes clean-up easy.