For the yoghurt and mint dressing, mix all of the ingredients and a pinch of salt together in a bowl until well combined.
For the mint vinaigrette, melt the mint jelly in a saucepan over a medium heat, then slowly whisk in the lime juice and groundnut oil until emulsified. Stir in the chopped mint and season with salt and freshly ground black pepper.
Cook the runner beans in a pan of boiling salted water for 2-3 minutes, or until tender. Drain the beans and refresh in iced water; once cool, drain again.
Place the knob of butter in a frying pan along with enough water to cover the base of the pan. Season with salt and freshly ground black pepper and warm gently until the butter has melted. Cook the prawns in the mixture, turning once or twice, for 2-3 minutes, or until cooked through.
Mix the beans, potatoes, red onion, sesame seeds and prawns together in a large bowl. Season with salt and pepper, drizzle with a little of the mint vinaigrette and mix until well combined.
Spoon the salad onto serving plates and drizzle with the yoghurt dressing and an extra spoonful of the mint vinaigrette.