For the dough: 1) In a medium bowl whisk together the whole-wheat flour, plain flour, caster sugar and salt. Add the butter and, using two knives or a pastry cutter, cut the butter into the flour mixture until you get a pebbly, coarse texture.
2) In a small bowl combine the buttermilk and ice water. Using a fork, gradually mix the buttermilk mixture into the flour mixture. Pat the dough into a 10cm round and wrap in cling film. Refrigerate for 30 minutes.
For the pear tart: 1) In the meantime, preheat the oven to 220°C / gas mark 7.
2) Peel the pears, core them and cut into 1/2cm slices. In a large bowl toss the pear slices with the lemon juice. Sprinkle in the cornflour, brown sugar and cinnamon and toss until the pears are evenly coated. Set aside.
3) On a lightly floured surface, roll the chilled dough into a large circle about 23cm in diameter. Line a baking tray with baking paper, and draping the dough over the rolling pin, transfer to the prepared baking tray. If the dough breaks at all patch it up with your fingers.
4) Arrange the pears in a mound in the center of the dough, leaving a 5cm border. Fold the border over the filling. It will only cover the pears partially and does not need to be even.
5) Bake the tart for 15 minutes, and then reduce the oven temperature to 180°C / gas mark 4, keeping the tart in the oven all the while, and bake for another 40 minutes, until the pears are tender and the crust is golden brown.
6) In a small bowl stir together the honey and boiling water to make a glaze. When the tart is done remove it from the oven and brush the honey glaze all over the top of the fruit and crust. Transfer to a plate to cool slightly.
7) Cut into six wedges and serve warm or a room temperature.