Tarragon Chicken Salad Sandwiches

Ingredients

• One 3-to-4-pound chicken
• Kosher salt and freshly ground black pepper
• Tarragon Mayonnaise
• 1 large egg (see note)
• 2 teaspoons Dijon mustard
• 1 teaspoon vinegar, preferably white balsamic vinegar
• ¼ cup fresh tarragon leaves
• ½ teaspoon kosher salt
• Freshly ground black pepper, to taste
• 1 cup neutral oil (such as vegetable oil)
• Lemon juice, as needed (optional)
• Quick-Pickled Onions
• 1 small red onion, thinly sliced
• Kosher salt
• 1 tablespoon white balsamic vinegar
• Assembly
• 8 slices of bread, such as Soft Sandwich Bread , sliced ½ inch thick
• Arugula or other greens