In a large skillet, heat olive oil over medium heat. Add onions, sugar and 1/2 tsp of salt, toss. Reduce heat to medium-low and cook til onions are golden, stirring. (15-20 mins) Remove from heat and cool. Stir in garlic.
In medium bowl, combine the cooled onion mixture, sour cream, mayonnaise, remaining salt and pepper. Cover and chill one hour to blend flavors.