Make the marinade by mixing together the sugar, salt, soy sauce and cornflour slurry. Mix in the minced beef, allow to marinate for 10 minutes.
Heat a large frying pan with 3 tablespoons oil. Add the potato and fry until browned, about 4-5 minutes. Put into a large saucepan with just enough water to cover. Bring to the boil and cook on medium heat for 10 minutes.
Heat the same frying pan, with the residual fat from the potatoes over high heat. Stir-fry the beef until 80% done, about 3-4 minutes. Drain away the fat and set aside.
Heat the same frying pan with 1 tablespoon oil over high heat. Stir-fry the garlic until aromatic, 2-3 minutes. Add the onion and salt and stir-fry until soft, 3-4 minutes. Add the baby sweetcorn and minced beef to the pan and mix well.
Add all of the ingredients into the same saucepan with the potatoes, adjust the seasoning with soy sauce, black soy sauce and oyster sauce. Stir in the mushrooms and stir for 4-5 minutes to heat through. Garnish with spring onions and serve.
Black soy sauce can be purchased in Chinese/Oriental speciality stores.