Preheat the oven to 200°C, gas mark 6. Grease an ovenproof dish that the pears will fit into snugly. Arrange the pears in the dish and scatter with the stem ginger and brown sugar. Pour over 600ml water and cover with foil. Cook for 1 hour 30 minutes, basting every half an hour, until the pears are tender but still hold their shape.
Meanwhile, combine the flour, ground ginger and a pinch of salt in a bowl. Rub in the butter with your fingertips until the mixture starts to clump together. Stir in the demerara sugar and hazelnuts. Chill in the fridge until ready to use.
Once the pears are tender, discard the foil and half the poaching liquid. Top with the crumble and bake for 15 minutes.
Remove from the oven and scatter over the dark chocolate. Serve warm with cream on the side, if you like.