Heat a large nonstick skillet coated with cooking spray over medium-high heat. Add onion; sauté 2 minutes. Reduce heat to low; cover and cook 20 minutes or until golden brown, stirring frequently. Remove from heat; cool.
Combine 1 tablespoon sour cream and cream cheese, stirring well. Add remaining sour cream, mayonnaise, and remaining ingredients, and stir well to combine. Stir in onion. Cover and refrigerate overnight.