1 pound/455 g carrots, cut into 3/4-inch/2 cm dice
1 large sweet-crisp apple, such as Fuji, cored and cut into 3/4-inch/2 cm dice
1 large yellow onion, cut into 3/4-inch/2 cm dice
Preheat oven to 450°F/230°C. Arrange a rack in the upper half of the oven and another in the lower half. Lightly coat a 9 by 13-inch/25 by 35 cm baking pan with oil.
In a small bowl, combine the mustard and maple syrup. Spread the mixture all over the pork. Place the pork in the prepared pan and pour any remaining mustard mixture over the top. Sprinkle all sides of the tenderloins with salt and pepper and set aside.
In a medium bowl, combine the carrots and 2 tablespoons of the oil, tossing to coat. Spread the carrots on a large, rimmed baking sheet. Place the pork on the upper rack and the carrots on the lower rack and roast for 20 minutes.
Meanwhile, in the same medium bowl, combine the apple, onion, and the remaining 2 tablespoons of oil, tossing to coat.
Remove the carrots from the oven and add the apple and onion (leaving the pork in the oven). Return the vegetables to the oven and continue cooking until an internal thermometer in the pork reads 140°F/60°C for medium rare, about 10 minutes. Transfer the pork to a cutting board and let it rest, covered loosely with foil, for 10 minutes.
Meanwhile, toss the carrot mixture and continue roasting until tender, about 10 minutes. Add salt and pepper to taste.
Slice the pork on an angle and arrange on plates or a platter. Spoon the carrot mixture on top or alongside. Drizzle with any accumulates juices and serve hot.