- Mix onions with butter, olive oil and thyme
- Caramelize onions slowly in wood oven until deep, dark brown. Remove from oven and cool to room temperature.
- Purée onions in blender or food processor with buttermilk.
- Add sour cream, mayonnaise and sherry vinegar
- Puree until very smooth, and add more buttermilk if necessary.
- Adjust acidity with more vinegar, and season with salt and pepper