Make the pretzel buns. Put the water, oil, sugar, salt and onion granules in your bread machine container. Top with the flours and the yeast (making sure the yeast does not touch the liquid ingredients). Use the dough setting to mix and prove the dough.
Preheat the oven to 200C (400F). Tip the dough onto a lightly floured worktop and deflate. Let it rest for five minutes. Form into a log shape and divide into six equal sections if making large buns or up to nine if making smaller ones.
Flatten each piece and then fold the edges in towards the centre. Flip over seam side down and place the palms of your hands around the roll. Use your hands to shape each roll it into a smooth ball.
Place the rolls on a large tray lined with baking paper, spaced apart. Cover loosely with greased cling film and leave them to rise for about 20-30 minutes.
While the buns are rising, bring a large pot of water to the boil. Stir in the bicarbonate of soda – take care as this will cause the water to bubble and rise in the pot. Once the soda has dissolved, lower the heat so that the water is barely simmering. Lower two buns at a time into the pot using a slotted spoon and cook for about 30 seconds on each side. Place them back on the tray and continue with the remaining buns.
Brush the buns with beaten egg and sprinkle with poppy seeds or salt flakes. Use a very sharp knife to slice twice on the diagonal. Bake for about 15-18 minutes or until the rolls are nicely browned. Leave to cool before serving.
To make the burgers, combine all the ingredients in a large bowl, kneading with your hands until everything is thoroughly combined. Use a burger press to form the burgers or shape with your hands. Chill in the fridge for 30 minutes.
Heat a little oil in a large non-stick frying pan. Fry the burgers, two at a time, over medium heat for about 4-5 minutes on each side. Because the burgers contain pork it is important to make sure they are fully cooked before serving. They should feel springy to the touch and be nicely coloured. Drain on a plate lined with kitchen paper.
Serve the burgers in the pretzel buns with fresh lettuce leaves, sliced tomatoes, cheese (if desired), ketchup and yellow mustard and topped with a few slices of Baxter's red onion.