Warm your oil in a wok or large skillet over medium heat until it shimmers slightly. Add the minced garlic and let it sizzle for about 30 seconds until fragrant and just starting to turn golden — don't let it brown or it'll taste bitter.
Toss in the mushrooms and chilies together. Stir-fry for 2-3 minutes, letting the mushrooms release their moisture while keeping some bite. They should shrink slightly but still hold their shape — you want them tender but not soggy.
Pour in the oyster sauce, soy sauce, sugar, and white pepper all at once. Stir everything together quickly so the sauce coats each mushroom piece. The sugar will help the sauce cling and balance the salty elements.
Kill the heat and immediately fold in the green onions. They'll wilt slightly from the residual heat but keep their bright color and fresh crunch. Taste and adjust seasoning if needed, then serve right away while everything's still hot.