Preheat the oven to 425ºF. In a mixing bowl, whisk together the creme fraiche, egg and mustard. Finely grate the zest from the lemon into the bowl, add ½ teaspoon salt and whisk to combine.
Line a baking sheet with parchment. Lightly flour a work surface and roll the puff pastry out to form a rectangle, about 13-by-11-inches and about ¼-inch thick. Transfer the dough to the prepared baking sheet. With a sharp knife, lightly score a ½-inch border around the edges of the puff pastry (be careful not to cut all the way through). Dock the center of the pastry all over with a fork.
With a spoon or spatula, evenly spread the creme fraiche mixture all over the center of the pastry, staying inside the borders. Bake until the pastry is golden brown and puffed around the edges, and the crème fraiche mixture is set, 25 to 30 minutes. (Check for air bubbles in the pastry once or twice during baking time; poke any that form gently with a fork to deflate.) Let cool on the baking sheet on a wire rack for at least 20 minutes.
Meanwhile, toss the shallot rings, vinegar and ½ teaspoon salt in a small bowl.
To serve, evenly layer the cucumber slices over the creme fraiche mixture. Season lightly with salt. Blanket the salmon over the cucumbers. Drain the shallots and sprinkle them over the salmon, followed by the capers and dill. Season lightly with pepper. Cut into squares and serve.