Preheat the oven to 425°. In a bowl, whisk the vinegar with the mustard, scallion, parsley and 1/4 cup of the olive oil. Season the vinaigrette with salt and pepper.
Heat a rimmed baking sheet in the oven. Cut the turnips in half through the stems; quarter them if large. In a large bowl, toss the turnips with the remaining 1/4 cup of oil and season with salt and pepper. Spread the turnips on the preheated baking sheet and roast for about 18 minutes, until tender. Transfer the turnips to a platter and let cool. Drizzle with the vinaigrette and serve.