PreparationFor horseradish sour cream: Whisk potatoes, if using, sour cream, horseradish, dill, parsley, and lemon juice in a small bowl; season with salt and pepper. For assembly: Spread horseradish sour cream on bread and top with smoked salmon, radishes, roe, if using, dill, and parsley. Season with pepper.
PreparationFor horseradish sour cream: Whisk potatoes, if using, sour cream, horseradish, dill, parsley, and lemon juice in a small bowl; season with salt and pepper. For assembly: Spread horseradish sour cream on bread and top with smoked salmon, radishes, roe, if using, dill, and parsley. Season with pepper.