Combine brown onion, chilli, fresh coriander, parsley, preserved lemon, lemon juice, garlic, paprika, cumin, ground coriander, turmeric and olive oil in the bowl of a food processor and pulse until combined but mixture still has some texture. Season with salt and pepper. Transfer chermoula to a large shallow dish. Add chicken and toss to coat. Place in the fridge, covered, for at least 30 minutes to marinate (overnight is better).
Meanwhile, using a vegetable peeler, cut the sweet potato into long, thin strips. Heat vegetable oil to 170°C in a deep frying pan over medium heat (oil is ready when a piece of sweet potato starts bubbling and popping as soon as you drop it in). Cook sweet potato, in batches, for 1-2 minutes or until it becomes firm and crisp. Using tongs, transfer crisps to a plate lined with paper towel
Preheat a chargrill over high heat. Cook chicken for 4 minutes each side or until chicken is cooked through (reduce heat to medium once chicken is sizzling so the marinade doesn’t burn).
Combine lime juice, sour cream and red onion in a small jug. Season with salt and pepper. Toast buns until golden. Top bun bases with lettuce, chicken, avocado, sweet potato crisps and a drizzle of lime sour cream dressing. Replace bun tops to serve.
Serving Size 4