Wash the choy sum, making sure to wash any mud that has accumulated in the overlapping parts of the stem at the base, and cut it into 7 cm wide pieces.
Slice the garlic and set aside it with the choy sum.
Wash the prawns lightly and pat dry thoroughly with paper towels.
Prep the tomatoes by removing the stems and making a cross-cut where the head was.
Blanch the tomatoes in boiling water for 10 seconds, then immediately cool them in cold water.
Peel, then halve the tomatoes horizontally, remove the seeds, and cut them into 1 cm cubes.Tip: Remove any remaining hard green-white parts of the tomatoes. Finely chop the onion, ginger, and garlic.
Prepare the potato starch mix with the water.Tip: Having all these things prepared in advance means you can quickly whip up the dish when guests arrive–simply stir-fry away! Heat 1 tbsp vegetable oil in a wok or frying pan, add the prawns, and fry briefly for 30 seconds, then remove the prawns from the pan.Tip: If your frying pan is not non-stick, add oil to the pan and place it on high heat before use, so that the prawns cook nicely without sticking to the bottom. However, be careful when heating the pan as it can be dangerous! When hot enough to produce sufficient smoke, turn off the heat and wait until the oil temperature has cooled to a reasonable level (maybe a few minutes) and then add the prawns.Tip: Be careful not to overcook the prawns as they’ll toughen and lose flavour. Tip: Keep the liquid and oil from the prawns when they are cooked and use them together later. Add 3 tbsp of vegetable oil in the wok over medium heat, add the garlic, ginger, and chili bean paste (Toban Djan), and stir-fry until fragrant, about 1 minute.
Once the garlic and ginger release their delightful aroma, add the onions and continue to stir-fry for about 5-6 minutes until the onions are mashed.Tip: The key is to gently coax out the sweetness of the onions. Don’t be too rough with them. Add the tomatoes, increase the heat, and stir-fry until the water is absorbed and they lose their shape, roughly 6 minutes.
Season with 1 tbsp oyster sauce and a pinch of salt, then add the prawns and stir-fry to coat for about 1 minute.
Finish off by quickly mixing in the potato starch (or cornstarch) mix to thicken the sauce.Tip: If starch is not available, cornflour can be substituted. However, it is less soluble and more absorbent than starch, so in that case, reduce the amount of cornflour by about 3/4 and dissolve it thoroughly in water before use.