Step 1: Shred the Chicken. Begin by finely shredding 500g of cooked chicken meat using your hands or a mixer. Aim for a texture that is smooth with no large chunks.

Step 2: Make the Sauce. In a mixing bowl, whisk together 160g of whole egg mayonnaise, 160g of sour cream, 2 teaspoons of Dijon mustard, 1 tablespoon of lemon juice, ½ teaspoon of onion powder, ¼ teaspoon of garlic powder, ¾ teaspoon of kosher salt, and ¼ teaspoon of black pepper. Continue whisking until the mixture is smooth and creamy.

Step 3: Combine Filling. In a large bowl, combine the shredded chicken with the prepared sauce and add in 1 finely chopped celery stalk, 1 finely sliced green onion, 75g of roughly chopped walnuts, 3 tablespoons of finely chopped cornichons, and 1 tablespoon of finely chopped dill. Stir gently until all ingredients are well coated in the sauce.

Step 4: Prepare Bread. Take 20 slices of white sandwich bread and spread a thin layer of salted butter on one side of each slice. Set the prepared slices aside as you start assembling your sandwich.

Step 5: Assemble Sandwiches. Place about 100g of the chicken mixture on one slice of bread (buttered side down) and cover it with another slice (buttered side up). Trim the crusts off each sandwich for a neat presentation.

Step 6: Chill. Once assembled, place the sandwiches in the refrigerator for 15-30 minutes.

Step 7: Slice and Serve. After chilling, remove the sandwiches from the refrigerator. Slice each sandwich into halves or quarters and serve cold on a platter.