1. In a pan, melt the butter then slowly cook the chopped onion until soft and almost opaque.
2. Add the sage, and cook for a little longer.
3. Add the breadcrumbs, mix together over a low heat until combined. The breadcrumbs should soak up all the butter.
4. Remove from the heat, and allow to cool.
5. Mix in the sausage meat, and then the chopped chestnuts.
6. Fold them in gently and mix well.
7. Place the stuffing mix into a suitably sized buttered baking dish and bake in a preheated oven for 15 minutes or until slightly brown on top at 160 degrees centigrade.