Put the sweet potato in a microwaveable bowl, cover with cling film and cook on high for 3 mins until soft. Heat the oil in a non-stick frying pan and fry the onion with a big pinch of salt until soft, around 5 mins. Add the garlic and spices and cook for 2 mins more, then set aside. Blitz the tofu with the cashews in a blender, then tip in the sweet potato and fried onions. Season and blitz again until combined. Shape into four burger patties and set aside in the fridge on a lined baking tray while you make the slaw.
Make the slaw by combining the ingredients in a bowl, then set aside until needed. When you’re ready to fry the patties, mix the mustard with the soy milk in one bowl, then mix the remaining coating ingredients in another bowl. In turn, take each patty and dip into the flour mixture, then into the soy milk mixture, then back into the flour mixture. Repeat this again so you get a really nice crust on the patties.
Pour the vegetable oil into a large frying pan over a medium-high heat - you want the oil to come about 2cm up the side of the pan. Once hot, use a spatula to carefully add the patties and gently fry for 1 min on each side until crispy, then leave to drain on kitchen paper. Serve the burgers in burger buns, topped with the slaw and drizzled with the mayo.