Peel the skin of daikon radishes and cut into long thin strips. Bring about 600ml water to boil; add the dakon radishes to cook for 2 minutes, drain, set aside.
Cut the heads from the prawns, and rinse the heads. Deshell and devein the prawns, rinse, set aside.
Heat the wok over high heat, add vegetable oil, ginger and spring onion, stir fry for 1 minute. Add the daikon, stir fry for 1 minute. Add chicken stock, 300 ml water, salt and white pepper, cook for 5 minutes. Add prawn heads and cook for 5 minutes, add the prawns; cook for 2 more minutes then add MSG to taste (if using).
Remove the prawn heads and ginger from the soup before serving.