Combine 300g plain flour, 1 tablespoon sugar and 1/4 teaspoon salt in a large bowl. Cut in butter and baking fat with a knife until mixture resembles coarse crumbs. Stir in ice water, 1 tablespoon at a time, until mixture forms a ball. Divide dough in half and flatten each half into a 12cm disc. Wrap in cling film and chill at least 1 hour or up to 8 hours (overnight).
Combine 200g caster sugar, 3 tablespoons flour, cinnamon, ginger, nutmeg and 1/4 teaspoon salt in large bowl. Add apples and pears and toss to coat well.
Preheat oven to 220 C / Gas 7. Position 2 of the oven racks in the bottom section of the oven. Place a sheet of foil (that's bigger than your pie dish) on the bottom rack to catch drips.
Roll out 1 piece of dough into a 30cm round on a lightly floured work surface with a floured rolling pin. Line a 23cm pie dish with pastry and trim overhang to 1cm with a sharp knife. Roll out remaining dough in same manner and cut into 10 strips (2cm wide) for a lattice.
Spoon apple and pear filling into pastry case. Brush pastry rim with some of egg wash. Dot filling with butter. Lay 5 strips of dough parallel across pie, about 2cm apart, letting excess hang over rim. Lay remaining strips diagonally across pie in opposite direction. Press ends firmly to rim of pie, trimming excess dough. Crimp edges with your fingers or a fork. Brush with remaining egg wash. Sprinkle with remaining tablespoon sugar.
Bake pie in lower third of oven for 15 minutes, then reduce temperature to 190 C / Gas 5 and bake until juice in centre of pie starts to bubble, about 45 minutes more. (Loosely cover pie with a sheet of foil if browning too quickly.) Transfer pie to a wire rack and cool for 2 hours.