Sweet potato burgers

Name: Sweet potato burgers
Ingredients: 140g long-grain brown rice; 850g sweet potatoes, peeled and cut into small cubes; 3 tbsp olive oil; 1 tsp ground cumin; 2 tbsp sunflower seeds; 1 small red onion, very finely chopped; thumb-sized piece ginger, peeled and finely grated; 1 red chilli, deseeded and finely chopped; small pack basil, leaves finely shredded, stalks discarded; 1 ripe avocado, stoned, peeled and diced; 2 tbsp lime juice; 4 brioche burger buns; 8 round lettuce leaves, washed and dried; 25g sweet potato crisps, or ready-salted potato crisps; ½ punnet mustard cress, snipped