Homemade French Onion Dip

This is a recipe from my friend Zoe-and it is AMAZING.  Next time you need to bring an appetizer to a party, make this yours!  These notes are hers-'This says slice onions but I dice/chop them before cooking to safe a step.  I also substitute 1 cup plain yogurt for half of the sour cream, and use a mix of red and vidalia onions'.  I've followed her modifications every time I make it.

2-3 TBSP unsalted butter
2 Vidalia onions, sliced into rings
2 large shallots, sliced into rings
3 cloves garlic, minced
2 cups sour cream
1 cup mayonnaise
1 TSP celery salt
1 TSP Worcestershire sauce
kosher salt & fresh ground pepper to taste

In a large saute pan over medium high heat, add butter.  When butter is melted, add onions and saute stirring occasionally until golden brown and caramelized, about 35 minutes.  Add shallots and saute for 15 minutes more until onions and shallots are dark brown.  Add garlic and saute for another 5 minutes.  Remove from heat and let cool for 5 to 6 minutes, then chop into 1/4 inch pieces.  Set aside to cool to room temperature.

Meanwhile, in a medium bowl, combine sour cream, mayonnaise, celery salt, Worcestershire, salt and pepper.  Fold in onion mixture.  Chill at least 1 hour of overnight (preferably), prior to serving.  Garnish with fresh chopped chives if desired.