Preheat oven to 350°F. In a large pot, cover the sweet potatoes with water and bring to a boil. Boil until tender. Drain and peel the potatoes and place in a large mixing bowl. Mash well. Add the olive oil, milk, sherry, brown sugar, orange juice, nutmeg, cloves, cinnamon and salt. Beat well with a mixer. Spoon mixture into an oiled, ovenproof gratin dish. Bake until the potatoes are hot, about 25 minutes. While the soufflé is cooking, make the meringue. In a clean bowl, whip egg whites until almost stiff. Gradually add the sugar and continue to beat until soft peaks form. During the last five minutes of cooking the soufflé, top with the meringue mixture. Return to oven and remove when the meringue is lightly browned. Garnish with the sliced oranges.