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Heat the oven to 200C/180C fan/gas 4. Lay the pears out in a medium ovenproof dish that fits them snugly. You don’t want large gaps between the pears or you won’t have enough topping to cover them. Scatter over the sugar and pour over the lemon juice and vanilla. Mix using your hands to coat the fruit well.
Tip the flour into a large bowl and rub in the butter using your fingertips until you achieve a coarse breadcrumb texture. Stir in the sugar and hazelnuts, then scatter this over the pears. Try not to pack it down too tightly, so that it remains crumbly when cooked. If you have any crumble topping left over, cook this next to the crumble in a small ovenproof dish (you can scatter it over the bowls of crumble later). Bake for 40-50 mins until golden and the pears are cooked through. Serve with custard.