Crack the walnuts, blanch the kernels in lightly-salted water, dry them on a dish towel and peel carefully. Then crush them adding small amounts of milk to obtain a thick smooth cream. Warm the stock in a large saucepan, add the walnut mixture and the cream. Check the seasoning, add some pepper and bring to the boil for a few minutes. Serve hot with croutons fried in butter.