Heat the 2 tsp oil in a large wok over medium heat. Add snow peas, capsicum and salt. Stir-fry until crisp-tender, about 3 minutes. Transfer to a plate.
Heat remaining oil in the same pan. Add prawns, spring onion and chilli flakes. Stir-fry 1½ minutes. Add garlic and ginger. Stir-fry 1 minute. Add soya sauce and lemon juice. Stir-fry until prawns are curled, bright pink and cooked through, about 1 minute.
Add snow peas and capsicum to pan. Stir-fry just long enough to heat through, about 30 seconds. Stir in lemon zest; serve immediately.