Peel the pears, rubbing them with lemon juice to stop them discolouring, then cut them in half and discard the stalks. Scoop out the core and pips with a teaspoon.
Melt the butter over moderate heat. As it starts to sizzle, add the sugar and pears and let them colour lightly. As they soften, let the sugar caramelise here and there. Tip the pears and their juices into a 1.5-litre baking dish. Set the oven at 180C/gas mark 4.
Make the crumble: rub the butter into the flour with your fingertips or use a food processor. When the mixture looks like coarse breadcrumbs, stir in the sugar and oats. Add a tbsp of water or milk and shake until it forms gravel-sized lumps.
Chop the chocolate into small pieces then fold it through the crumble. Tip the mixture over the pears, leaving the surface quite rough. Bake for 40-45 minutes, until lightly coloured.
