Preheat the oven to 180C.
Pan fry the chopped onion in the butter for 5 minutes, until slightly softened.
Mix all of the other ingredients together, along with the onions and butter that they were cooked in.
Stir until everything is thoroughly mixed (you may need to use clean hands!) and spoon into a shallow dish. (Don't smooth the top, as the lumps and bumps give a good texture.)
Bake for 45 minutes or until cooked all the way through.
