Heat oven to 200C/180C fan/gas 6. Toss the sweet potatoes with the oil, smoked salt and polenta in a large bowl or ziplock bag, massaging them well to coat. Spread out onto two large baking trays, then roast for 45 mins until tender and crisp, turning the trays halfway so that the fries cook evenly.
Meanwhile, blitz all the ingredients for the dip together with some seasoning to a smooth green sauce – this can be made in the morning and kept in the fridge. Serve alongside the sweet potato fries.