Peel the cucumbers and cut them in half lengthwise. Scoop out the seeds with a teaspoon and dice the cucumbers.

Transfer the cucumbers to a blender or food processor. Add the dill, mustard, vinegar, sour cream and pepper. Purée until it is completely smooth. Transfer to a mixing bowl and refrigerate until chilled.

Heat a small skillet over medium heat. Add the mustard seeds and toast them until they turn gray, taking care to not burn them. You may need to partially cover the pan to keep the seeds from jumping out. Remove them from the heat and let cool.

Just before serving, stir half the chopped radishes into the soup. Ladle the soup into shallow rimmed plates. Sprinkle with the remaining radishes, sprigs of dill and the mustard seeds.