Make the brittle. Heat the sugar and measured water in a frying pan until it dissolves, stirring gently from time to time. Add the nuts, then increase the heat and boil the syrup for 5 minutes, without stirring, until pale golden. Quickly tip the mixture on to a greased baking sheet and leave to cool. Break the brittle in half, then crush half in a plastic bag with a rolling pin.
Whisk the egg whites until very stiff, then gradually whisk in half the sugar until thick and glossy. Whisk the egg yolks in a second bowl with the remaining sugar until very thick and pale and the mixture leaves a trail. Fold in the lemon rind and rose water, if using, then the yogurt and crushed brittle, then the egg whites. Pour into a plastic box and freeze for 4–5 hours until semi-frozen and firm enough to scoop.
Cook the pineapple slices on a hot barbecue or preheated griddle pan for 6–8 minutes, turning once or twice until browned. Divide between the serving plates, top with spoonfuls of semifreddo and decorate with broken pieces of the remaining brittle.