Preheat the oven to 180 c and grease an ovenproof dish large enough to hold the crumble.
Mix the pears, brown sugar, lemon zest and juice and half the chocolate together in a bowl and put into the ovenproof dish.
To make the crumble add the flour , butter, sugar and hazelnuts to a food processor and pulse until just combined. Add the remaining chocolate and stir together.
Spoon the crumble over the pears and press down lightly. Bake for 35-40 minutes until golden and bubbling.