Using a sharp knife, score the fat on the top of the roast in a crosshatch pattern, cutting just into the meat layer so the marinade can penetrate it. Place the roast in a baking pan and season on all sides with 2 teaspoons salt and 1 teaspoon pepper. Pat the garlic all over the roast, then coat on all sides with the pimenta moida, rubbing the seasonings into the slits on the top of the roast. Sprinkle the cilantro all over the roast, then drizzle with 1⁄2 cup of the oil, and rub all the seasonings to evenly distribute. Cover with plastic wrap, and refrigerate overnight.

Allow the roast to come to room temperature, 11⁄2 to 2 hours. Preheat the oven to 425°F. Transfer the roast to a baking pan large enough to hold it and the potatoes in one layer. Drizzle some of the marinade juices over the top of the roast. Place the potatoes in the first pan and toss with the remaining marinade. Add the potatoes and marinade to the roast, arranging the potatoes around it; sprinkle the potatoes with the remaining 1⁄4 teaspoon salt and pinch of pepper, and drizzle with the remaining 2 tablespoons oil. Roast the pork until lightly golden and the slits on top of the roast have opened up, 25 to 30 minutes.

Reduce the oven temperature to 350°F and cook until an instant-read thermometer inserted into the center of the roast registers 140°F, 35 to 40 minutes longer. Remove from the oven and set aside to rest at least 15 minutes before serving. (If the potatoes are not yet golden around the edges and cooked through, transfer the roast to a plate, then return the potatoes to the oven and roast for 10 to 15 minutes longer while the meat rests.) Carve the roast into thin slices and serve with the potatoes, drizzling any pan drippings over the pork.