2 tablespoons vegetable oil
1 tablespoon butter
4 Vidalia onions, sliced into 1/4-inch rings (about 5 cups)
2 large shallots, sliced into 1/8-inch rings (about 1/2 cup)
1/4 cup scallions, sliced into 1/4 inch pieces
1 cups sour cream
1 cup mayonnaise
1 tablespoon dijon mustard
1 teaspoon celery salt
1 teaspoon Worcestershire
1 teaspoon freshly ground black pepper
1 teaspoon salt
In a large sauté pan over medium high heat, add oil and butter. When butter is melted, add onions and sauté stirring occasionally until golden brown and caramelized, about 35 minutes. Add shallots and sauté for 15 minutes more until onions and shallots are dark brown. 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, mustard, scallions, celery salt, Worcestershire, salt and pepper. Fold in onion mixture. Chill at least 1 hour or overnight. Let come to room temperature prior to serving.