Caramelize the sugar with the butter. Pour the caramel into a small, buttered, rectangular cake form.
Pour 3 1/3 cups (800 ml) of the water, plus the sugar, in a pot and leave them to boil.
Dissolve the semolina in the remaining water. Pour it in a thin trickle and stir until it begins to thicken.
Leave to cool a bit and pour 1/2 of it onto the caramel. Sprinkle with fruits and cover with the remaining 1/2.
Leave at room temperature to cool, then in the fridge for a few hours to harden completely.
Finally, invert onto a plate, sprinkle with coarsely ground walnuts and caramel topping.