Combine all the ingredients listed under marinade including the chicken and set aside for 10-15 minutes.
Whisk together oyster sauce, soy sauce, vinegar, black pepper, water and half the cornstarch and set aside.
Once the chicken has marinated, toss it in the remaining cornstarch.
Cook chicken pieces, in batches on high heat till golden (approx 3 min) - tossing the pan so all the sides of the chicken pieces become golden. Remove the pieces from the pan and set aside.
Add the remaining oil to the pan on high heat, and add the onions, and bell peppers. Fry for two minutes, just to soften slightly then add the ginger, garlic and tomato, cook for another minute.
Add the chicken pieces and sauce mixture to the pan. Mix and simmer till the sauce thickens and coats the chicken well. Serve with rice or steamed vegetables.