Preheat the oven to 180 C, 170 Fan, Gas 4. Peel the pears rubbing each one with a lemon half to stop them discolouring. Cut in half, discard stalk and scoop out the core with a teaspoon.
Melt the butter in a shallow frying pan. When it starts to sizzle tip in the pears and the sugar. Let the fruit colour lightly and the sugar caramalise here and there , leaving them patchily golden.
Tip the pears and their juice into a 1.5 lite ovenproof dish.
Make the crumble by rubbing the butter into the flour with your fingertips or using a food processor. When the mixture looks like fine breadcrumbs stir in the demerara sugar and oats.
Add 1 tbsp water and shake the crumble mixture until some of it sticks together in gravel sized lumps. Chop the chocolate into small pieces about the size of course gravel then fold through the crumble.
Tip the crumble over the fruit, leaving the surface quite rough and making no attempt to pack it down. Bake for 45 minutes until lightly coloured.