Separate the lettuce leaves and wash and dry well. Cut out the ribs if they are tough. Mix the mascarpone cheese with the chopped walnuts and the fresh dill. Spread onto the lettuce leaves and lay a strip of smoked salmon in the middle. Roll up the leaves and press down gently to give it some shape. Wrap in clingfilm. It will hold its shape better if you put it in the fridge for a while, and then you can slice it like maki.