2 tablespoons vegetable oil
2 medium onions, chopped (3 cups)
3/4 cup mayonnaise
3/4 cup sour cream
Salt and freshly ground black pepper
Finely chopped fresh chives for garnish
1. Heat the oil in a large skillet over medium-low heat. Add the onions, cover, and cook, stirring occasionally, until golden, about 20 minutes. Uncover and continue cooking, stirring often, until deep golden brown, 15 to 20 minutes. Cool completely.
2. Mix the mayonnaise and sour cream in a medium bowl. Stir in the onions. Season with salt and pepper. Cover and refrigerate until chilled, at least 2 hours, or up to 2 days. (If necessary, thin with milk before serving.)
3. Transfer to a serving bowl and sprinkle with chives. Serve chilled or at room temperature.