- 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.