Peel and slice the sweet potato, dice the onion and garlic and sauté in the melted butter. Add the potato slices and sauté a little. Pour in the stock and simmer for 10 minutes. Puree with the cutting stick and bring to the boil, switch off the stove, pour in the couscous and let it swell. Season with freshly ground pepper, fold in the sour cream and sprinkle with chives.