In a small skillet, saute onion in oil for 1 minute. Reduce heat to medium-low. Cover and cook for 8 minutes or until onion begins to turn golden brown. Add the green onions, salt, pepper and cayenne. Cook and stir for 2 minutes. Add garlic; cook 1 minute longer. Remove from the heat; cool to room temperature, about 15 minutes.
Meanwhile, in a small bowl, combine the sour cream, mayonnaise and cream cheese until smooth. Stir in onion mixture. Garnish with additional green onions. Cover and refrigerate for at least 1 hour. Serve with breadsticks and peppers.