Heat the oven to 200°C (180°C in a fan oven), gas 6.
Chop the prawns and the chicken to form and pliable mixture.
Stir in the chilli and the cumin and shape into four burgers.
Place them in a bamboo steamer and cook for 8 minutes.
Heat the butter in a pan and fry the onion.
Remove and pat dry with kitchen paper.
Cut the rolls in half and toast them.
Mix together the yogurt and lemon juice with salt and ground black pepper and spread on the bottom halves of the rolls.
Top with the cucumber and chard leaves and place a burger on top.
Add some mint leaves and the fried onion rings and place the top halves of the rolls on top.