1 large sweet potato, peeled, halved, and sliced 1/2 inch thick
3 large parsnips, peeled, halved, and cut into 2-inch pieces
1 tablespoon finely grated peeled fresh ginger
3 tablespoons olive oil, plus more for serving
2 tablespoons light-brown sugar
1/4 teaspoon cayenne pepper
1 pork tenderloin (about 12 ounces), excess fat and silver skin removed
Coarse salt
1 bunch watercress, trimmed
Lime wedges, for serving

Preheat oven to 475 degrees. On a rimmed baking sheet, toss sweet potato, parsnips, and ginger with oil and spread in an even layer. Mix together brown sugar and cayenne and rub all over pork; add to sheet. Season pork and vegetables with salt and roast until an instant-read thermometer inserted in center of pork reads 145 degrees, 20 to 22 minutes. Let pork rest 10 minutes before serving. Toss vegetables with watercress, drizzle with olive oil, and season. Thinly slice pork and serve with any accumulated juices from sheet, the salad, and lime wedges.