1) Combine sugar, vinegar and ginger in a small pot and bring to a boil, dissolve sugar and turn off heat, reserve.
2) In a frying pan, heat the oil over medium-high heat. Saute the onions, chilli peppers, mushrooms and garlic for 3 to 4 minutes, then add the prawns and toss for 3 to 4 minutes more until pink and firm. Douse the pan with the sweet vinegar, and season with salt and pepper, to taste.
Serve with cold lettuce for wrapping the hot-sweet prawns.