In very large skillet, heat butter and oil over medium heat til butter is melted. Add onions, salt, and sugar. Reduce heat to medium low and cook for 40 to 50 minutes or til onions are deep golden brown and caramelized,stirring constantly. transfer 1/3 cup of the onions to small airtight container; cover and chill for at least 4 hours or up to 3 days. Stir cayenne pepper and garlic into onions, remaining in skillet. Cook for 5 minutes more. Remove from heat. Cool slightly and coarsely chop.
In large bowl, use potato masher to mix sour ream, softened cream cheese and mayonnaise. Stir in onion/garlic mixture. Transfer to airtight container. Cover and chill at least 4 hours or up to 3 days
Let stand at room temperature for 30 minutes before serving. Top sour cream mixture with the 1/3 cup caramelized onions. If desired, garnish with sliced green onions. Serve with potato chips and/or crackers. Serves 10