Make the marinade by mixing together the sugar, soy sauce and cornflour slurry. Mix in the beef, let marinate for 15 minutes.
Heat a large frying pan with 1 tablespoon oil over high heat. Stir-fry the beef until browned and only 50% cooked, about 2-3 minutes. Set aside.
Heat the same pan with butter over medium heat. Saute the mushrooms until cooked, about 2-3 minutes. Transfer to a serving dish.
Heat the same pan with 1 tablespoon oil over high heat. Saute the garlic until aromatic, 2-3 minutes. Add the onion and salt; cook until soft, 3-4 minutes. Stir in the peppers and mix well.
Return the beef back into the pan with sugar, oyster sauce and water. Mix well and serve on top of the mushrooms.
King oyster mushrooms are also known as king trumpet mushrooms or French horn mushrooms. If your local supermarket doesn't stock them, you can purchase them online. Alternatively, you can substitute in any other mushrooms you prefer.