For the walnut crumble
Combine the rice and walnut milk and walnuts and allow to stand overnight. Place in a food processor and blend until smooth and creamy.
Add the vanilla, tahini paste and half the sugar. Blend and pour into a mixing bowl.
Drain the chickpeas, reserving the liquid. Pour the liquid into the bowl of an electric mixer and whip until soft peaks form. Gradually add the remaining sugar a little at a time to form a stiff, glossy meringue. Gently fold the meringue into the walnut mixture. Pour into a loaf tin lined with baking paper and freeze overnight.
To make the walnut crumble, rub all the ingredients together to form a rough mixture. Spread onto a baking sheet and bake at 180°C, gas mark 4, for 15 minutes or until golden.
Remove the semifreddo from the freezer 20 minutes before serving.
Serve in slices, topped with the walnut crumble.