Heat oil and butter in a pan over medium heat. When the butter has melted, add the onions to the pan and fry, turning occasionally, until they are light to golden brown and caramelized (approx. 35 minutes). Add the shallots and fry for about 15 minutes until the onions and shallots have a dark golden brown color.
Remove from heat and let cool for 5 - 6 minutes. Then cut into small pieces and set aside to cool the onions to room temperature.
Meanwhile, mix the sour cream or sour cream, mayonnaise, celery salt, Worcestershire sauce, salt and pepper in a larger bowl, add the onions and fold in well.
Chill in the refrigerator for at least 1 hour, but better for 3 hours.
The dip can also be mashed. If it`s too thick, add some milk.