Clean the strawberries and cut them in half lengthwise. Melt the butter. Pour 1/2 cup (120 ml) cold water over the gelatin and leave it to swell.
Grind the walnuts together with the biscuits. Add the butter and stir. Put the mixture in a cake form with a ring, press down toward the bottom. Leave it in the fridge.
Set aside a few strawberries for decoration, mash the rest. Heat the strawberry puree and once bubbles begin to form along the surface, remove from the heat. Strain the gelatin and add it to the strawberry puree.
Mix the cream with the mascarpone and powdered sugar and beat to a a creamy mixture. Add the strawberry puree, then pour this mixture over the layer in the ring. Leave it for 3 hours to harden. Take it out of the form, decorate with the strawberry halves and serve.