Melt the butter in a frying pan and add the onion on low to medium heat. Add some salt and pepper and fry for 5 minutes or until soft.
Add the remaining ingredients, including more salt and pepper. Mix and fry for 1 minute. Brush a small oven-proof dish with butter and add the mixture. Spread it out evenly and bake it in the oven for 30-35 minutes.
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