Put the semolina and sugar in a saucepan. Pour in the milk, add the butter, lemon zest and mixed spice, then bring to the boil over a moderate heat, stirring occasionally. Immediately reduce the heat and simmer the semolina gently for 15 minutes, stirring from time to time, until the mixture has thickened. Remove the pan from the heat and allow to cool slightly.
Pour the semolina into four ramekins or individual serving dishes. Sprinkle the walnuts over the top, then drizzle with maple syrup or golden syrup. Serve warm.
Not all supermarkets sell semolina so you may need to try your local health-food shop. In supermarkets it is often stocked near the health-food items.