Radish, kohlrabi, and potato salad with sour cream and dill dressing

8 ounces new potatoes, scrubbed and cut into bite-sized pieces (about 2 cups);

8 ounces small kohlrabi, peeled and cut into bite-sized pieces (about 2 cups);

2 tablespoons white wine vinegar or rice vinegar;

1⁄4 cup sour cream (regular or reduced fat);

1 teaspoon wholegrain Dijon mustard;

2 tablespoons finely chopped fresh dill;

2 tablespoons finely chopped shallot or spring onion;

Kosher salt and ground black pepper, to taste;

4 ounces radishes, trimmed and thinly sliced (about 1 cup).

Place the potatoes and kohlrabi in a large pot and cover with cold water. Bring to a boil, add 1⁄2 teaspoon kosher salt per cup of water, reduce the heat, and simmer only until tender when pierced with the tip of a knife, about 15 minutes. Drain in a colander. Return the vegetables to the pot and set aside for a few minutes so they can steam dry. Sprinkle the vinegar over the vegetables and let cool to room temperature, stirring occasionally. They will absorb the vinegar as they cool.
Stir together the sour cream, mustard, dill, and shallot in a large bowl. Season generously with salt and pepper. Stir in the potatoes, kohlrabi, and radishes. Cover and refrigerate until chilled, at least 30 minutes. Just before serving, stir and taste for seasoning.
What else works? You can replace some or all of the cooked kohlrabi with raw seedless cucumber.