Smoked Salmon Smørrebrød

Name: Smoked Salmon Smørrebrød
Ingredients: 1/2 fennel bulb—halved lengthwise, cored and very thinly sliced; 1/2 Granny Smith apple, cored and very thinly sliced; 3 teaspoons fresh lemon juice; 3 tablespoons crème fraîche; 1 1/2 tablespoons skyr (Icelandic yogurt) or Greek yogurt; 1 tablespoon finely grated fresh horseradish; Pinch of sugar; Kosher salt; Unsalted European-style butter, softened, for spreading; 4 slices of dense rye bread; 1/2 pound sliced smoked salmon; Small dill sprigs and flaky sea salt, for garnish