Start by preparing the king oyster mushrooms. Rinse them under cold water and pat dry. Slice them into ¼-inch rounds or strips, or use a fork to create a shredded texture by scoring and pulling them into strands.
In a small mixing bowl, whisk together the oyster sauce, soy sauce, sugar, black pepper (if using), and the cornstarch slurry (if using). Stir until the mixture is smooth and set aside.
Heat a large skillet or wok over medium-high heat. Add the cooking oil and let it heat until shimmering. Toss in the garlic and ginger and stir-fry for about 30 seconds to 1 minute, or until aromatic and just beginning to turn golden.
Add the mushrooms to the pan and stir-fry for 3 to 5 minutes. Keep the heat high to drive off moisture quickly and encourage browning and caramelization. The mushrooms should soften and reduce slightly in size, developing a golden exterior.
If using bell peppers or other vegetables, stir them in at this point and cook for another 1 to 2 minutes until they're vibrant and slightly tender.
Pour the sauce mixture into the pan, stirring continuously to ensure the mushrooms are evenly coated. Let it cook for another 1 to 2 minutes, allowing the sauce to thicken and glaze the mushrooms.
Turn off the heat. Finish the dish with a garnish of fresh spring onions or scallions and, if desired, a light drizzle of sesame oil for extra aroma. Serve immediately with a bowl of steamed rice for a delicious, satisfying meal.