Bring a saucepan of water to a boil. Add the snow peas and blanch for 1 minute. Drain, briefly refresh in cold water, then drain again. Slice lengthwise and place in a mixing bowl. Add the mango and most of the mint, reserving some mint for garnish. Set aside. For the dressing: Whisk together the oil, mustard and vinegar and season with salt and pepper.
Heat a wok over medium heat. Add the vegetable oil and saute the shallots and ginger until caramelized, 3 minutes. Add the prawns and stir-fry until just cooked, 2 minutes. Stir in the lime juice to deglaze the pan, then season with salt and pepper. Add the prawns to the mixing bowl with the snow peas and mint, then pour over the dressing and toss until well combined. Transfer to a serving platter, garnish with the remaining mint and serve.