Cold-Fashioned Potato Salad

INGREDIENTS

• 3 tablespoons cider vinegar
• 1 tablespoon chopped fresh tarragon
• 1/4 cup chopped parsley
• 3 tablespoons fine chopped cornichons
• 1/2 teaspoon freshly ground black pepper
• 1/2 cup small dice red onion
• 2 1/2 pounds red potatoes, large diced
• 1 teaspoon kosher salt
• 1/2 cup thinly sliced celery
• 1/2 tablespoon very thinly sliced garlic
• 3/4 cup mayonnaise (homemade if possible)
• 1 teaspoon mustard powder