Add the cold butter and rub between your fingers/palms into the dry ingredients until only pea size chunks remain. Chop the walnuts roughly and and add to this mix, squeezing chunks of crumble together to vary the size and make some larger chunks too. Mix the dark chocolate through the crumble mix.
Distribute the crumble over the fruit and bake for about 1 hour or until bubbling, golden brown on top and the pears are tender. This crumble benefits from a longer time in the oven to really caramelise the pears - if you feel that the top is browning too fast, cover it with a piece of aluminium foil.