1 | Heat olive oil in a large saute pan over medium-low heat. Add onions and cook, covered, stirring often, until onions are soft and translucent, about 8-10 minutes. |
2 | Uncover, reduce heat to low and continue to saute, stirring occasionally, until onions are completely soft and beginning to caramelize, about 15 minutes. Add scallions and continue to saute 5 minutes more. |
3 | Add sugar and continue to saute, stirring constantly, until onions achieve a golden caramel color, about 5-7 minutes. Add balsamic vinegar and stir to combine. Cook long enough to allow the vinegar to evaporate. Transfer to a mixing bowl and let cool. |
4 | Add sour cream, mayonnaise and chives to cooled onion mixture. Season to taste with salt and pepper. Chill in refrigerator for 3 hours or more. (Can be made up to 2 days before serving.) |
5 | When ready to serve, top with diced tomato. Serve with tortilla chips or crostini. |