How to Make It
Preheat oven to 400°F. Tightly wrap beets in aluminum foil and place on a baking sheet. Roast until beets are tender when pierced with a knife, 30 to 35 minutes.
Meanwhile, heat 1 tablespoon oil in a large ovenproof skillet over medium-high. Season pork with 1 teaspoon salt and ½ teaspoon pepper; add to hot skillet and cook until golden, about 3 minutes per side. Transfer skillet to oven and bake pork until cooked through, 16 to 18 minutes. Transfer pork to a cutting board; let rest 5 minutes.
Whisk together vinegar and remaining 3 tablespoons oil, ½ teaspoon salt, and ¼ teaspoon pepper in a large bowl. Remove cooked beets from oven, carefully unwrap foil, and let cool 2 to 3 minutes. Add beets, arugula, and parsley to dressing and toss to coat. Sprinkle salad with pistachios. Thinly slice pork; serve with beet salad.