Heat a large pan on medium heat and gently fry onions in butter and oil for 1-2 minutes. Add spices and salt, reduce heat and cook for a further 15-20 minutes, stirring occasionally.
Remove from the heat and allow to cool.
In a bowl combine onion mixture, yogurt and mayonnaise and mix well. Taste and adjust seasoning with salt and pepper. Refrigerate for 2 hours.
