How to Make It
Fill a medium bowl with ice water. Add the sliced fennel and apple and let sit until crisp, about 10 minutes. Drain well and pat dry. In a medium bowl, toss the fennel and apple with 1 teaspoon of the lemon juice.
Meanwhile, in a small bowl, mix the crème fraîche with the skyr, horseradish, sugar and the remaining 2 teaspoons of lemon juice. Season the horseradish crème with salt.
Spread butter on the rye bread and arrange the smoked salmon on top. Dollop the horseradish crème on the salmon, then top with the fennel-apple salad. Garnish with dill and sprinkle with flaky sea salt; serve.