Make the marinade by mixing together the sugar, salt, soy sauce and cornflour slurry. Mix in the chicken, let marinate for 20 minutes.
Meanwhile, blanch the bean sprouts in boiling water for 30 seconds. Drain and set aside.
In a small bowl, mix together the oyster sauce, sugar and water to make the sauce. Set aside.
Heat a large frying pan with 1 tablespoon oil over high heat. Stir fry the garlic and garlic sauce until fragrant, about 2-3 minutes. Stir in the onion and salt and cook until soft, 3-4 minutes. Add the chicken and stir fry until the meat turns opaque, 3-4 minutes.
Add the sauce, mix and allow to heat through, 1-2 minutes. Close the lid and simmer for 3 minutes. Stir in the capsicum, cook for 1-2 minutes. Add the bean sprouts and cornflour slurry and stir until the sauce is thickened. Serve.