Salmon Burgers with Chive and Lemon Mayonnaise

Name: Salmon Burgers with Chive and Lemon Mayonnaise
Ingredients: 1 large egg; 2 tablespoons Dijon mustard; 1 lemon , zested plus 1 tablespoon juice; 2 tablespoons capers , chopped if large; 1/2 teaspoon salt; 1/4 teaspoon black pepper; 1/2 cup parsley leaves, packed; 1/4 red onion , cut into 1-inch pieces; 1 1/4 pounds skinless salmon fillet , cut into 1-inch pieces; 3/4 cup panko; 2 tablespoons olive oil; 3/4 cup mayonnaise; 1 1/2 tablespoons lemon juice; 2 tablespoons chives , finely chopped; 1 tablespoon drained horseradish; 1 teaspoon Dijon mustard; Pinch black pepper; 4 burger buns; 4 slices tomato; 4 slices red onion; 4 leaves lettuce