To make the pastry, place the flour in a large bowl with the salt and rub in the butter until the mixture resembles fine breadcrumbs. Stir in the sugar, then bring the pastry together with the egg (it may also need 1 tablespoon of cold water if it’s a little dry). Wrap the pastry in cling film and chill for at least 30 minutes (or overnight is fine).
Preheat the oven to 180°C (350°F/gas mark 4). Line 2 large baking sheets with non-stick baking paper.
Peel, core and slice the pears and put them in a bowl. Toss them in the lemon juice, then toss with the flour, brown sugar and cinnamon.
On a lightly floured board, cut the pastry into 4 even-sized pieces. Roll each piece of pastry out to a rough circle about 15cm (6in) in diameter and no more than 5mm (¼in) thick. Carefully transfer to the lined baking sheets. Arrange the pears in the middle of each one and scatter the hazelnuts on top, leaving a 2.5cm (1in) border. Using the sides of the baking paper to help lift the pastry, fold the sides up and over the pears. Brush with the beaten egg and bake for 30–40 minutes, until the tarts are cooked through and golden.