To make cornstarch (corn flour) slurry, combine together cornstarch (corn flour) and water in a small bowl.
To make the sauce, heat oil in a frying pan over medium-high heat.
Add garlic, ginger and green chilies. Saute, stirring, for 2 minutes or until aromatic.
Add onion. Saute, stirring occasionally, for about 3 minutes or until soft.
Add spring onion whites (scallions) and bell peppers (capsicum). Saute, stirring occasionally, for about 2-3 minutes or until soft.
Add water, soy sauce, vinegar, chili paste (optional) and ground white pepper. Stir to combine. Season with salt.
Cover with a lid. Simmer for about 4 minutes.
Add cornstarch (corn flour) slurry. Stir to combine.
If the mixture is too thick, add a little water.
Cover and simmer for another 4-5 minutes.
Garnish with chopped spring onions (scallions) (optional).
Add this sauce to Indo-Chinese stir fries and noodles.