1) Preheat the oven to 180°C / gas mark 4.
2) Heat the butter or oil in a small frying-pan over medium heat. Add the onion and saute until translucent, about five minutes. Remove from the heat and transfer to a large bowl.
3) Add all of the remaining ingredients to the bowl and mix together until thoroughly combined.
4) Pour the mixture into a greased 3L casserole dish. Bake for 20-25 minutes or until bubbly.
Let the casserole stand for a few minutes before serving.