Place the tomato ketchup / tomato sauce, sriracha, soya sauce and vinegar in a small jug or bowl. Stir and set aside.
Heat oil in a large wok / medium sized pot (or Dutch oven). The same pan can be used that was used for frying the chicken, just remove the oil and leave only 2 tablespoons in the pan.
Once the oil is hot, add the freshly minced garlic and whole red chilies. Saute for a minute, and then add the cubed onions and capsicum.
Sauté for around 2 minutes, and then add the tomato sauce mixture that was kept aside.
Add the chicken stock, along with the red chili flakes, white pepper powder and salt. Add only a little salt as the other ingredients are already quite salty.
Let the mixture come to a boil, and then turn heat to simmer.
Add the cornflour slurry and cook for 2 – 3 minutes till the gravy thickens and turns glossy.
Add the fried chicken to the gravy and mix through. Cook for a few minutes till the chicken is heated through. The gravy will thicken a little more due to the fried chicken batter. If you want, you can add more chicken stock at this stage, and adjust seasonings.
Once the chicken is heated through, turn off the heat. Garnish the chicken Manchurian with sliced spring onions and serve hot with fried rice.