Preheat the oven to 220°C and line two baking trays with baking paper.
Cut the sweet potatoes into evenly sized matchsticks, about 5 mm thick. Place in a bowl or sealable bag, add the arrowroot or tapioca flour and salt and toss or shake to coat evenly. Drizzle over the oil and toss or shake again.
Arrange the sweet potato matchsticks in a single layer over the prepared trays, leaving space between them. Bake for 15 minutes, then flip with a spatula and cook for a further 10–15 minutes, or until the fries are crispy and starting to turn brown in spots.
Meanwhile, make the cashew sour cream. Drain the soaked nuts and rinse them really well, then transfer to a high-speed blender or food processor together with all the remaining ingredients and blitz for 1-2 minutes, or until beautifully smooth.
To make the guacamole, mix all the ingredients in a bowl until well combined (you can make it as smooth or as chunky as you like).
Pile the sweet potato fries onto a large platter and top with generous dollops of the guac and cashew sour cream. Sprinkle over the coriander leaves, season well with salt and serve with the lime cheeks for squeezing, if you like.