1 (3-pound) bottom-round beef rump roast
1 cup coarsely chopped fresh flat-leaf parsley
1 cup thinly sliced scallions (from 1 bunch)
2/3 cup mild olive oil
2 tablespoons drained capers (in brine) plus 2 tablespoons brine
Put oven rack in middle position and preheat oven to 500°F.
Pat roast dry and sprinkle all over with 2 teaspoons salt and 1/2 teaspoon black pepper.
Roast, fat side up, in a roasting pan 15 minutes. Reduce oven temperature to 325°F and continue to roast until thermometer inserted into center of meat registers 120°F, 35 to 45 minutes more. Transfer roast to a cutting board and let stand, uncovered, 15 minutes.
While roast stands, stir together remaining ingredients, a slightly rounded 1/4 teaspoon black pepper, and a pinch of salt.
Thinly slice meat across the grain and serve with sauce.