Preparation

• Heat the oil in a 12-inch skillet over medium-low heat. Add the onions and rosemary and cook, stirring occasionally, until the onions begin to brown, about 15 minutes.
• Add the vinegar and cook, scraping up the browned bits on the bottom of the pan, until the vinegar has evaporated, 1 minute. Transfer the onions to a food processor and let cool for 10 minutes. Add the cream cheese, sour cream, mayonnaise, and cayenne and pulse until mostly smooth. Season to taste with salt and pepper. Let the dip stand at room temperature for at least 30 minutes before serving so the flavors can develop. If necessary, thin with a little water.

Make Ahead Tips

The dip can be made up to one week ahead, covered and refrigerated.