Make the marinade by mixing together the sugar, salt, soy sauce and cornflour slurry. Mix in the chicken, let marinate for 15 minutes.
In a small bowl, mix together oyster sauce, sugar and sesame oil to make the sauce. Set aside.
Heat a large frying pan with 1 tablespoon oil over high heat. Stir fry the garlic and ginger until fragrant, 2-3 minutes. Stir in the onion and salt and fry until soft, 3-4 minutes. Add the chicken, stir fry until all sides are evenly browned, 3-4 minutes. Stir in the shimeji mushrooms and black fungus, stir to mix.
Add in 1/2 tablespoon water and allow to heat through, 1-2 minutes. Reduce the heat to medium and simmer for 5 minutes or until the chicken is fully cooked. Stir in the sauce and mix to combine, serve hot.
If you'd like a thicker sauce, add a little bit of cornflour mixed with water at the end.