Preheat the oven to 180˚C, gas mark 4. Put the flours, oats, almonds, demerara sugar, cinnamon, 100g butter and the salt in a food processor and pulse together until just combined. Peel and core the apples and pears, then cut both into thin wedges. Put the fruit, muscovado sugar, vanilla seeds, 50g finely chopped butter and lemon zest and juice in a bowl; mix with your hands until the fruit is evenly coated.
Transfer to a rectangular ovenproof dish about 27cm x 17cm, then cover with the crumble mixture. It shouldn’t be too neat, as an uneven surface will result in more colour and crunch. Bake for 30 minutes, until golden. Serve while still hot, with a scoop of ice cream.
