Put the olive oil and rosemary leaves into a large, heavy bottomed saucepan and place over high heat. When the rosemary begins to sizzle, about 1 minute, add the couscous. Stir to coat well and cook for 2 minutes, stirring frequently. Add the chicken stock, salt and pepper, lower the heat and let simmer until all of the stock has been absorbed, about 20 minutes. Remove from the heat, fluff with a fork and transfer to a chafer. Garnish with fresh sprigs of rosemary around the sides of the dish.