Mix the prawns with the fajita seasoning and the oil, set aside. Mix the beans with the lime juice, half the coriander and a good pinch of seasoning. In a separate bowl, stir the jalapeño into the mayo.
Fry the prawns to just pink, remove from pan, set aside.
Split the buns in half, fill each with a quarter of the beans, then the mango. Top with the prawns, drizzle over the mayo, scatter over the remaining coriander and serve with the lime wedges.