Heat the butter and olive oil in a large skillet over medium heat. Add the sliced onions and cook, stirring occasionally, for about 10 minutes or until they are caramelized and golden brown.
Sprinkle sugar over the onions and continue cooking for another 5 minutes, stirring occasionally, to help the onions caramelize further.
Remove the skillet from heat and let the onions cool for a few minutes.
In a medium bowl, combine the sour cream, mayonnaise, and Worcestershire sauce. Mix well until fully combined.
Add the cooled caramelized onions to the sour cream and mayonnaise mixture. Stir until all the ingredients are evenly distributed.
Season with salt and black pepper to taste. Adjust the seasonings according to your preferences.
Cover the bowl with plastic wrap and refrigerate for at least 1 hour to allow the flavours to meld.
Serve the French onion dip with your favourite chips, crackers, or vegetable sticks.