Steam the asparagus, covered, 3 minutes or until tender. Rinse asparagus with cold water, and pat dry. Combine the asparagus, greens, and radishes in a large bowl. Combine the dressing and mustard in a small bowl, and stir well with a whisk. Add the vinaigrette and feta to salad, tossing gently to coat.
Note: You can prepare the greens, asparagus, and radishes up to an hour ahead. Combine them in a large bowl; cover with a paper towel and chill.