Finely shred the cooked chicken meat using your hands or a mixer, ensuring there are no large chunks. Set aside in a mixing bowl.

In a separate bowl, whisk together the mayonnaise, sour cream, Dijon mustard, lemon juice, onion powder, garlic powder, kosher salt, and black pepper until smooth and creamy.

Gently fold the shredded chicken into the creamy sauce, then add the celery, green onion, walnuts, cornichons, and dill. Stir until evenly combined.

Spread butter on one side of each slice of day-old bread. Heat a skillet over medium heat and place the bread buttered side down.

Spoon about 100 grams of the chicken filling onto one slice of buttered bread and top with another slice, pressing gently to hold together.

If desired, chill the assembled sandwiches in the refrigerator for 15–30 minutes for easier slicing.

Cut the sandwiches into halves or quarters and serve cold, optionally with a side salad or potato chips.