For the celery and cucumber cold soup with smoked char tartare, first puree all the ingredients for the soup very finely in a bar blender or with a hand blender. Season to taste with salt and pepper. If necessary, strain through a fine sieve and then refrigerate.
For the tartare, mix all ingredients together not too vigorously and season with salt and pepper.
Wash the mini cucumbers and remove the ends. Using a peeler, peel lengthwise into thin slices. Lay the slices on a work surface, top with a tarragon leaf, add some tartar and roll up.
Slice radishes into thin slices.
Place the cucumber-celery cold dish in a deep plate. Place the cucumber rolls inside and garnish the celery-cucumber cold dish with smoked char tartare with the radish slices and some cress.