1. In a large heavy based saucepan heat the oil and butter together over a medium heat.
2. Add the onions and cook until caramelised, keep stirring so they don’t stick. Add the celery salt, a good grind of pepper, the Worcester sauce and thyme sprigs.
3. The onions will take approximately 20 minutes or so to caramelise and cook down.
4. Meanwhile mix together the yoghurt and cream cheese in a bowl.
5. When the onions are browned and cooked and delicious, discard the thyme sprigs and mix the onions into the yoghurt. Adjust seasoning and serve with crackers or crisps.