On a rimmed baking sheet, toss the sweet potato, parsnips, and ginger with oil and spread in an even layer.
Mix together the brown sugar and cayenne, rub all over the pork, and add pork to the sheet pan.
Season pork and vegetables with salt, and roast until an instant-read thermometer inserted into the center of the pork reads 145 degrees F, 20-22 minutes.
Let pork rest 10 minutes.
Meanwhile, toss the roasted vegetables with the watercress, drizzle with oil, and season with salt.
Slice the pork and serve with any accumulated juices from the sheet, the salad, and lime wedges.
Recipe Notes
Recipe adapted from One Pot Martha Stewart Living, retrieved January 2015.