Slice the potatoes halfway through lengthwise to create a slit, careful not to pierce through the center. Spiralize on Blade A and add to a medium bowl with the olive oil. Season with salt and pepper and toss well to combine.
Heat a large grill pan over medium-high heat. Once hot (flick water into the pan and if it sizzles, it’s ready), add the potatoes and cook, tossing occasionally until cooked through and charred, about 10 minutes.
While potatoes cook, in a large bowl, whisk together the yogurt, Dijon mustard, half of the chives, and season with salt and pepper.
Once potatoes are done, add to the bowl with the yogurt dressing and toss well to combine. Transfer to a serving bowl or platter and serve garnished with remaining chives and extra pepper.