PREHEAT THE OVEN TO 425 degrees . On a baking sheet, toss the parsnips and fennel with 3 tablespoons olive oil; season with salt and pepper. Rub the pork with olive oil, season with salt and pepper and place on the baking sheet. Roast, turning once, until the pork is cooked through, about 25 minutes. In a large skillet, boil the vinegar over medium-high heat until reduced by half; transfer to a bowl. In the same skillet, heat 1 tablespoon olive oil over medium-high heat; add the arugula and cook until wilted. Season with salt and pepper. Slice the pork and place over the arugula. Drizzle with the sauce and serve the roasted vegetables alongside.