Preparation In a small bowl, whisk together the mayonnaise and buttermilk. In a mortar and pestle, mash the garlic and onion with the fine sea salt to create a paste, or use a large heavy knife to mince and mash the garlic and onion with the salt into a paste. Add the garlic-onion paste and dry mustard to the buttermilk mixture and whisk well to combine. Add the chives, parsley, dill, and freshly ground black pepper and whisk well to combine. Season the dressing with additional fine sea salt. DO AHEAD: The dressing can be prepared ahead and refrigerated, in an airtight container, up to 1 week.
Preparation In a small bowl, whisk together the mayonnaise and buttermilk. In a mortar and pestle, mash the garlic and onion with the fine sea salt to create a paste, or use a large heavy knife to mince and mash the garlic and onion with the salt into a paste. Add the garlic-onion paste and dry mustard to the buttermilk mixture and whisk well to combine. Add the chives, parsley, dill, and freshly ground black pepper and whisk well to combine. Season the dressing with additional fine sea salt. DO AHEAD: The dressing can be prepared ahead and refrigerated, in an airtight container, up to 1 week.