Wash all vegetables. Remove the stem from the tomato. Halve the cucumber lengthways and scrape out the seeds with a teaspoon. Cut off the leafy green tops and roots from the radishes.
Cut the tomato, cucumber, and radishes into bite-sized pieces. Slice the spring onion into rings. Arrange all vegetables in a shallow bowl.
Add the sour cream, smoked salmon, and salt and pepper to taste to a blender. Blend until smooth, about 1 minute. If using fresh herbs, add them and blend for about 15 seconds until combined.