In a heavy bottomed saucepan, cook together the chopped onions, butter, vinegar, 1 tsp garlic salt, and sugar.. Cook over medium high until the butter melts, then lower the heat to low. Cook uncovered for 30-45 minutes stirring occasionally until browned and caramelized.
Using a hand mixer whip together the sour cream, mayonnaise, cream cheese, Worcestershire sauce, 1 ½ tsp garlic salt and pepper.
Mix in the caramelized onions by hand until fully combined
Place into an air tight container and chill overnight.
Garnish with chopped green onion or fresh chives, if desired.