Preheat the oven to 170˚C/gas mark 3 and lightly grease a 900g loaf tin.
Melt the butter, sugar and treacle in a medium saucepan, stir through and remove from the heat. Cool slightly then add the buttermilk and eggs.
Sieve the flours, raising agents, spices and salt into a medium bowl. Add to the liquid ingredients in the pan in stages, stirring thoroughly after each addition until evenly mixed.
Pour the batter into the prepared loaf tin and bake on the middle shelf of the oven for 45 minutes, then cover with aluminium foil to prevent the top from scorching.
Bake for a further 10–15 minutes or until the loaf has risen and feels firm to the touch. If in doubt, insert a metal skewer into the centre and if it comes out clean the cake is done.
Place the cake on a wire rack and allow to cool for a few minutes before turning it out and piercing the top in several places.
Make a clementine glaze by mixing together the icing sugar, clementine zest and juice, and lemon juice until you have a sticky icing. It shouldn't be too runny or the glaze will just pour right off the cake.
Drizzle the glaze over the ginger cake and sprinkle with pomegranate seeds and pistachios to decorate.