From Cook's Illustrated.
Ingredients
3/4 cup mayonnaise
1-1/2 tablespoons minced cornichons (about 3 large) plus 1 teaspoon cornichon juice
1 tablespoon minced scallion
1 tablespoon minced red onion
1 tablespoon capers, minced
Instructions
Mix all ingedients in a small bowl. Cover and refrigerate until flavors blend, at least 30 minutes. (Can be refrigerated for several days.)