Combine the yogurt, mint, cayenne pepper, and salt in a bowl.
Melt mint jelly in a saucepan and whisk in lime juice and groundnut oil until emulsified.
Stir chopped mint and seasoning into the vinaigrette.
Boil the runner beans for 2-3 minutes until tender, then refresh in iced water and drain.
Melt butter in a frying pan with enough water to cover the base.
Sauté the prawns in the butter mixture for 2-3 minutes until cooked through.
Toss beans, potatoes, onion, sesame seeds, and prawns in a bowl with a little vinaigrette.
Divide onto plates and drizzle with yogurt dressing and remaining vinaigrette.