Tarragon is a natural herb to use in spring dishes, since it is a perennial herb and one of the first to peek out of your garden. If tarragon is not to your taste, it can be easily replaced with chives or dill in the same measure.
For the dressing, puree all of the ingredients until smooth and season to taste. Chill until ready to serve (the dressing will keep for up to 4 days, refrigerated).
For the salad, trim the asparagus and blanch it for about 3 minutes in boiling, salted water, until tender. Drain and rinse to cool.
Arrange the salad greens on a platter, and top with the asparagus and radishes. Spoon the dressing over and serve.