Whisk together the crème fraîche and lemon juice in a small mixing bowl until smooth. Layer the slices of apple and cucumber on half of the slices of bread.
Fold the slices of smoked salmon and sit them on top. Spoon over some of the crème fraîche mixture.
Sprinkle over the chopped dill and sit the other slices of bread on top. Garnish with the halved slices of cucumber on top before serving.