Heat oil and add garlic. Fry till it becomes brown. Add sliced onion. Cover and cook till it becomes soft, no need to become brown. Add prawns and the vegetables. Fry for 3-4 mins or till they are crunchy tender. Add all seasonings/sauces and salt. Mix well. Cover and cook for 5 mins on low flame.
Dissolve cornflour in 1/2 cup water and add to the curry. Cook for 5 mins. Add cubed pineapple and mix well. Cook for 3-4 mins.
Garnish with chopped spring onion. Serve with steamed/boiled white rice or fried rice.
Notes
To make chicken or pork: Add chicken (cut into small pieces) or pork (thinly sliced) along with onion. Cover and cook on low heat till chicken/pork is tender. Then follow the rest of the recipe as mentioned above. To make fish, follow the same procedure as prawns. The original recipe doesnt use chilli powder and soya sauce. So you can omit it if you want. Fish sauce substitute: combination of 2 parts soy sauce and 1 part lime juice (please note that I havent tried this, but it's recommended in some sites) Please be careful while adding salt, since the fish sauce and soya sauce already has salt in it.