Gather your mise en place; get together all of the equipment needed for the recipe and gather your ingredients.

In a large bowl, combine the flaked whitefish and cooled mashed potatoes. Mix gently to keep some texture.

Add the mayonnaise, egg, Dijon mustard, lemon zest, lemon juice, dill, and green onions. Mix until evenly combined. Season with salt and pepper.

Stir in ¾ cup breadcrumbs, adding more if needed, until the mixture holds together and can be shaped.

Form into evenly sized patties (about 2 – 3 inches wide). Place on a tray and refrigerate for at least 20 – 30 minutes to firm up.

Peel the hard-boiled eggs. Separate the yolks and whites. Finely chop the egg whites and set aside.

In a bowl, mash the egg yolks with the Dijon mustard and apple cider vinegar until smooth. Slowly whisk in the olive oil to create a thick, emulsified sauce. Stir in the chopped egg whites, capers, cornichons, parsley, tarragon, and chives. Season with salt and pepper to taste.

Set up a breading station: one bowl with flour, one with extra breadcrumbs, one bowl with the 2 eggs, beaten well. 

Dip each fish cake into the flour, then beaten eggs, then coat in breadcrumbs, pressing lightly to adhere.

Heat a thin layer of oil in a skillet over medium heat. Fry the fish cakes in batches for 3 – 4 minutes per side, until golden brown and heated through. Transfer to a paper towel-lined plate and keep warm.

Serve the hot fish cakes with a generous spoonful of sauce gribiche on top or alongside. Garnish with extra herbs or lemon wedges if desired.