Shot through with tarragon, parsley, and scallions, this tangy sauce pairs beautifully with rich southern-fried sweetbreads. If you have any left over, use it as a salad dressing or as a dip for crudités.
1/2cupmayonnaise
1/3cupwell-shaken buttermilk
2tablespoonswhite-wine vinegar
1/4cupcoarsely chopped flat-leaf parsley
2scallions, white and pale green parts chopped and greens finely chopped (reserve separately)
1teaspoonanchovy paste
2tablespoonsfinely chopped tarragon
Pulse mayonnaise, buttermilk, vinegar, parsley, white and pale green parts of scallion, anchovy paste, and 1/4 tsp each of salt and pepper in a food processor until parsley and scallion are finely chopped. Transfer to a bowl and stir in scallion greens and tarragon. Chill until ready to use.