Preheat the oven to 400°. Cut the squash in half from stem to blossom end.
Cut the squash in half from stem to blossom end. Scrape out and discard the seeds. Salt and pepper the cavity and place cut side down on a baking sheet. Bake for 25-30 minutes, or just until the squash can be pierced with a fork.
While the squash cooks, make the stuffing. Melt the butter and sauté the onion until it's soft. Add the chopped apple and sauté a few minutes more, until the apple is completely covered in the butter. Remove from the heat. Add the remaining ingredients and mix well. Add salt and pepper to taste.
When the squash is ready, remove them from the oven and turn the oven to 350°. Immediately flip the squash cut side up. Using a light hand, stuff the squash cavities, allowing the stuffing to mound up a bit. Bake for 20 minutes or until the stuffing is getting crisp on top.