Preheat oven to 500˚F. Place the oven rack in the center of the oven.
Pat the roast dry with paper towels and sprinkle all over with 2 teaspoons of salt and 1/2 teaspoon of pepper. Set in a roasting pan with the fat side facing up. Roast for 15 minutes. Reduce the heat to 325˚F, and continue to roast for 30-45 minutes more, until a thermometer inserted into the center reads 120˚F. Transfer the roast to a cutting board and allow to rest, uncovered, for 15 minutes.
In a small bowl, combine the parsley, scallions, minced garlic, capers, caper brine, and olive oil. Add 1/8 teaspoon of salt and 1/4 teaspoon of pepper. Taste and adjust the seasoning if needed.
Slice the roast thinly across the grain, and serve with the scallion-caper sauce.