From chef Kevin Johnson of the Grocery in Charleston, South Carolina
5 pounds onions, very thinly sliced
12 thyme sprigs, picked and minced
6 ounces butter
Salt and pepper
1 cup buttermilk, plus more if needed
2 cups sour cream
1 cup Duke’s mayonnaise
¼-½ cup sherry vinegar
Preheat oven to 400 degrees. Add onions, butter, olive oil, and thyme to a nonstick dutch oven and cover with lid. Bake for 45 minutes, stirring every 15, until onions are deeply caramelized. Remove from oven and cool to room temperature.
In a blender or food processor, purée onions with buttermilk. Add sour cream, mayonnaise, and sherry vinegar and puree until very smooth, adding more buttermilk if necessary.
Adjust acidity with more vinegar, and season with salt and pepper.