In a bowl, mix the crème fraiche and the lemon zest. Add salt and pepper, and mix until combined.
Couper le concombre et les radis en fines tranches.
Spoon a spoonful of the mixture onto each of the (toasted if desired) sourdough slices, and top with sliced of radish and cucumbers.
Roll up slices of smoked salmon and place on top of the slices of bread. You might have to cut the salmon slices smaller to fit comfortably onto the slices of bread. Garnish with sprigs of dill.