Heat olive oil in a heavy-bottomed pan or Dutch oven over medium heat. Add onions and thyme. Let cook until onions are very soft and caramel in colour, about 40-45 minutes.

Add vinegar and Worcestershire sauce and let cook until slightly thickened, about 15 minutes. Season with salt and pepper and remove pot from the heat and allow to cool to room temperature.

Meanwhile, in the bowl of an electric mixer or with a hand mixer, beat together cream cheese, sour cream and mayonnaise until fluffy, about 5 minutes. Add cooled onions and celery salt and stir to combine.

Transfer to refrigerator to chill at least 2 hours and up to 2 weeks.

Serve with plain chips or fresh veggies.