Cut the potatoes into ½-inch cubes. Peel the parsnips and cut into ¼-inch-thick slices. Pour ½ cup water into a medium pot fitted with a steamer basket, and bring to a boil over high heat. Add the cubed potatoes and sliced parsnips, cover the pot, and lower the heat to medium.

Cook vegetables until tender, 10 minutes. Remove from the pot and set aside to cool for at least 10 minutes. Fill the same pot (without the basket) with water and bring to a boil over high heat. Add the edamame and lower the heat to medium. Cook for about 5 minutes, until heated through. Drain, and set aside to cool for 5 minutes.

In a large bowl, combine the cooled potatoes, parsnips, and edamame. Chop the scallions, both whites and greens. In a medium bowl, whisk together the chopped scallions, mayonnaise, yogurt, soy sauce, vinegar, salt, and pepper.

Add this dressing to the potatoes mixture, and mix gently to combine.