• 2 large shallots, chopped
• 1 cup (240 ml) champagne vinegar or white wine vinegar
• 1 cup (240 ml) dry white wine (such as a chardonnay)
• 3 tablespoons chopped fresh tarragon leaves
• 1½ cups (360 ml) store-bought mayonnaise
• ¼ cup (60 ml) shaken buttermilk
• ¼ teaspoon kosher salt

In a medium nonreactive saucepan, combine the shallots, vinegar, wine, and 2 tablespoons of the tarragon, bring to a boil, and cook until the mixture is reduced to ½ cup (120 ml), about 15 minutes. Let cool slightly.

In a food processor, combine the mayonnaise, buttermilk, and shallot reduction and process until smooth. Add the remaining tarragon and the salt and pulse a few times just to incorporate.

Transfer to a container with a tight-fitting lid and refrigerate for at least 30 minutes to allow the flavors to meld. The dressing will keep, tightly covered in the refrigerator, for up to 3 days.