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Heat oven to 200C/180C fan/gas 6. Cook the rice following pack instructions, then drain in a sieve. Spread the rice out on a baking tray to steam-dry – it needs to be as dry as possible before adding to the sweet potatoes.
Meanwhile, toss the sweet potatoes with 2 tbsp olive oil and the cumin. Season and spread out in a large roasting tin in a single layer. Roast for 30 mins until tender and coloured, but not too crisp. Meanwhile, put the sunflower seeds in a cup, cover with water and set aside to soak while the sweet potatoes cook.
Put the remaining olive oil in a small saucepan set over a low-medium heat. Add the onion with a pinch of salt and cook, stirring often, for 5 mins. Add the ginger and chilli, and cook for 5 mins more until very soft. Set aside.
Drain the sunflower seeds, pat dry on kitchen paper and tip into a large mixing bowl with the sweet potatoes. Crush gently using a masher or the back of a fork. Add the rice, onion mixture, half the basil and seasoning to taste. Continue to crush together until the mixture is well mixed but not smooth. Form into 4 x 10cm diameter patties. Can be made up to 48 hrs ahead – keep covered and chilled until needed and allow to sit at room temperature for 15 mins before cooking.
Put the patties on a baking tray lined with baking parchment. Roast for 30 mins until warmed through and beginning to brown.
Make the salsa in a bowl by combining the avocado with the lime juice, remaining basil and seasoning to taste. Heat the grill to medium and toast the cut sides of the burger buns for 1 min or until golden brown. Top each bun base with a couple of lettuce leaves. Put the patties on top of the lettuce, top with a spoonful of salsa, a few sweet potato crisps and a scattering of mustard cress. Press on the toasted bun tops and serve with any leftover crisps on the side.