Stir together the flours and salt in a large bowl.
In a jug, mix the sugar, yeast and roughly half of the water together. Set aside for 5 minutes until slightly bubbly. Make a well in the centre of the dry ingredients and pour the yeast mixture in. Add half of the remaining water to the bowl as well. Stir it all together with your hands, adding more water as needed*. Tip the contents of the bowl out onto a clean work surface and knead for about 10 minutes until smooth and firm.
Lightly oil the bowl, return the dough to the bowl and turn to coat. Cover with a clean tea towel and leave in a warm place to rise for an hour.
Punch the risen dough down and divide into 5 equal pieces. Shape each piece into a ball and flatten into a thick oval. Place onto a floured baking tray and cover with oiled cling film. Leave to prove for 30 minutes in a warm place.
Meanwhile, bring about 2 litres of water to the boil in a large pot. Stir the baking soda, salt and syrup/sugar into the water and leave to simmer. Oil or butter a large loaf tin and use the sesame or poppy seeds to 'flour' the tin (if you don't want to use seeds, just oiling the loaf tin is fine).
Preheat the oven to 350 F (200 C). Use a slotted spoon to drop one oval of dough into the simmering water - boil for 1 minute then flip and boil for 1 minute on the other side. Remove from the water with the slotted spoon then place into the loaf tin. Repeat with the other ovals of dough, lining them up inside the tin next to each other. Brush the cracks in between the dough ovals with beaten egg to ensure they stick together during baking. Brush the top of the loaf with beaten egg and sprinkle on more sesame seeds or poppy seeds. Bake for 30-40 minutes until dark golden. Leave to cool in the tin for a few minutes before turning out onto a cooling rack to cool completely before slicing.