Ingredients

• 6 large sweet potatoes (about 1 1/2 kg)
• 2 tsp oil, plus extra for the trays
• 2 red onions, finely chopped
• 1 tbsp ground cumin
• 1 tbsp ground coriander
• 340g can sweetcorn, drained
• small bunch coriander, chopped
• 200g polenta
• buns, salsa, onion and salad leaves, to serve