Roast walnuts in oven or skillet and chop. Place polenta, salt, and sultanas in a saucepan and gradually stir in apple juice. Bring to a boil, reduce heat and simmer for 30 to 45 minutes, stirring occasionally until mixture thickens. Stir in lemon zest and nuts. Simmer a few minutes longer and serve warm. For a sweeter pudding, use pure maple syrup.