In a medium-sized bowl, whisk together sour cream, mayonnaise, fresh dill, lemon juice, lemon zest, and a pinch of salt until well combined. If using capers, stir them in and refrigerate for 30 minutes.

Peel and cut russet potatoes into chunks, boil in salted water until tender (about 15 minutes), then drain and let steam dry.

Transfer cooled potatoes to a mixing bowl and mash until smooth and fluffy; avoid adding butter or milk.

Flake the smoked trout into small pieces for mixing with the mashed potatoes.

Combine cooled mashed potatoes, flaked trout, beaten egg, panko, dill, chives, parsley, Dijon mustard, lemon zest, salt, and pepper in a large bowl; mix gently.

Shape the mixture into 8 patties and coat each with additional panko breadcrumbs.

Refrigerate formed cakes on a baking sheet for 15-20 minutes to firm up.

Heat neutral oil in a skillet and fry potato cakes in batches for about 4 minutes on each side, until golden brown.

Serve fried cakes warm with chilled lemon dill sauce; garnish with lemon wedges and dill as desired.