6 large sweet potatoes (about 1.5kg/3lb 5oz)
2 tsp oil
2 red onions , finely chopped
2 red chillies , finely chopped (deseeded if you like)
1 tbsp ground cumin
1 tbsp ground coriander
340g can sweetcorn , drained
small bunch coriander , chopped
buns , avocado slices and red onion to serve
Heat oven to 200C/180C fan/gas 6. Pierce the potato skins and place on a baking tray. Bake for 45 mins until really soft. Remove from the oven and leave to cool. Meanwhile, heat the oil in a small pan, add the onions and chillies, and cook for 8-10 mins until soft. Leave to cool.
Peel the potatoes and add the flesh to a bowl with the chilli onions. Mash together with the spices until smooth. Using your hands, mix in the sweetcorn, coriander, half the polenta and some seasoning. Shape the mixture into 10 burgers; it will be quite soft. Carefully dip each one into the remaining polenta; dust off any excess. Place burgers on oiled baking trays and chill for at least 30 mins. You can wrap and freeze the burgers at this stage for another day.
Heat oven to 220C/200C fan/gas 7 and cook on oiled baking trays for 15 mins. Serve in buns with a dollop of salsa, red onion and sliced avocado.