How to Make It
Place a large rimmed baking sheet in oven. Preheat oven to 425°F. (Do not remove baking sheet while oven preheats.)
Place potatoes and radishes in a large bowl. Add 2 tablespoons of the oil, 1/2 teaspoon of the salt, and 1/4 teaspoon of the pepper; toss well to coat. Arrange potato mixture in a single layer on hot baking sheet. Bake in preheated oven 15 minutes, stirring after 10 minutes. (Do not clean large bowl.)
Meanwhile, combine asparagus, remaining 1 tablespoon oil, and 1/4 teaspoon each of the salt and pepper in large bowl; toss well to coat. Remove baking sheet from oven; stir potato mixture again. Add asparagus mixture, spreading in a single layer. Bake until potatoes are tender and asparagus is tender-crisp, about 5 minutes.
Whisk together mayonnaise, buttermilk, vinegar, parsley, chives, shallot, tarragon, mustard, and remaining 1/4 teaspoon each salt and pepper in a small bowl. Drizzle desired amount of dressing over potato salad. Serve warm or at room temperature. (Reserve remaining Herb Dressing for another use.)