Combine the cucumber, vinegar, olive oil and ¼ teaspoon of the salt and pepper together in a small bowl and set aside at room temperature for 30 minutes. Strain off the liquid.
Combine the cucumber, crème fraiche, chives, dill, lemon zest and remaining ¼ teaspoon of salt and pepper in a small bowl and stir to combine. Cover and refrigerate at least 1 hour.
Meanwhile, •preheat the oven to 350°. Arrange the bread slices on 1 or 2 baking sheets. Brush both sides of the bread generously with the oil. Top each with some cheese and season with salt and pepper.
Bake for 15 to 20 minutes, until golden. Rotate the baking sheet to 180° halfway through the cooking time.
Arrange the crostini on a serving platter. Top each with a slice of salmon, then top with a dollop of the cucumber mixture. Garnish with dill and lemon zest to serve.