Place the pears in a shallow ovenproof dish with the sugar, cinnamon and measurement water and stir together. Cover with foil and place in a preheated oven, 190°C (375°F), Gas Mark 5, for 5 minutes.
Meanwhile, make the crumble topping. Place the sugar in a food processor with the cinnamon, oats, flour and butter and pulse until the mixture resembles fine breadcrumbs. Alternatively, rub the butter into the dry ingredients with your fingertips. Stir the golden syrup into the topping mixture.
Remove the pears from the oven, uncover and scatter the topping over them. Return to the oven for a further 20–25 minutes or until bubbling and lightly browned. Serve warm with custard.