Preheat the oven to 180°C.
Finely chop the celery and onion.Roughly chop the chestnuts.
Cut the squash in half lengthwise, seeds were removed and scrape the flesh, leaving 2cm edge.
Heat a spoonful of olive oil in a frying pan, add the bacon and cook until crisp, then add the chopped onion, celery, squash flesh and the rosemary leaves. Stir in the chilli flakes and nutmeg, then season with salt and pepper.
Cook gently over low heat for about 5 minutes until the vegetables have softened.
Turn off the heat, add the egg, orange zest, chestnuts and enough breadcrumbs to hold together the stuffing.
Fill both squash halves, then put them together, forming the original, whole squash shape. Wrap in foil tightly then place it onto a baking sheet.
Bake for about 90 minutes - it’s done when you can easily push a skewer through.
Unwrap the squash and cut it into a 3cm thick slices. Serve warm with a salad or as a side dish.