Make the marinade by mixing together the sugar, salt, soy sauce and cornflour slurry. Mix in the chicken, let marinate for 20 minutes.
Meanwhile, in a small bowl, mix together the ketchup, Worcestershire sauce, salt, sugar, water, soy sauce and HP sauce to make the sauce. Set aside.
Heat a large frying pan with 1 tablespoon oil over high heat. Fry the chicken until evenly browned and 80% cooked, about 3-4 minutes. Set aside
In the same frying pan, heat 1 tablespoon oil over high heat. Stir-fry the garlic until fragrant, 2-3 minutes. Add the onion with salt and cook until soft, 3-4 minutes. Stir in the chicken and sauce, allow to heat through, 2-3 minutes. Serve hot.
If you like your sauce a little thicker, add a little bit of cornflour and water to the sauce at the end.