First, julienne the apple: halve it lengthwise, and using a melon baller, or pairing knife, carefully remove the core. Thinly slice each half of the apple, and then thinly slice the slices into matchsticks. Transfer to a bowl and toss with half of the lemon juice so the apple doesn’t discolor.
Cut the salmon slices into long strips about 1-inch wide. Place a few pieces of watercress and several matchsticks on the top of one end of a salmon strip. Roll up the salmon. Repeat until you have used all your salmon.
In a small bowl, stir together the crème fraîche, the remaining lemon juice and the fresh dill. Season with salt and pepper, to taste. Serve in a ramekin on the side for dipping.