Combine cornflour, eggwhite, 2 tablespoons water and ½ teaspoon salt. Add prawns, toss to coat and set aside for 10 minutes. To make Szechuan sauce, combine all ingredients.
Heat half of oil in a wok on high. Stir-fry sugar snap peas for 2 minutes, until bright green. Remove and set aside.
Cook prawns in remaining oil for 2 minutes, until opaque. Remove. Stir-fry onion and ginger for 10 seconds. Return prawns with Szechuan sauce and stir-fry for 30 seconds. Toss through sugar snap peas.