In a large heavy-based saucepan heat the oil and butter together over a medium heat. Add the onions and cook for around 20 minutes or until caramelised, keep stirring so they don’t stick.
Add garlic, celery salt, a good grind of pepper, Worcestershire sauce and thyme sprigs. Meanwhile mix together the yogurt and cream cheese in a bowl until smooth. When the onions are browned and cooked, discard the thyme sprigs and mix the cooled onions into the yogurt. Adjust seasoning and serve with crackers or crisps.