1/4 cup good quality Mayonnaise
1/4 cup chilled buttermilk
1/2 clove garlic, peeled
1/2 tablespoon finely chopped onion (brunoise)
Pinch sea salt, plus additional to taste
1/4 teaspoon dry mustard powder
1 T finely chopped fresh or dry chives
1 T finely chopped fresh flat-leaf parsley
1 T finely chopped fresh or dry dill weed
Pinch freshly ground black pepper

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.