Pour the oil into your wok or largest skillet and set over medium heat. Once the oil is hot but not smoking, add the minced garlic. Let it sizzle and dance around for about 30 seconds until it turns golden and releases that irresistible aroma — don't let it go brown or it'll turn bitter.
Toss in the mushrooms and sliced chilies together. Keep everything moving with your spatula for 3-4 minutes. The mushrooms will release some moisture at first, then start to caramelize slightly around the edges. You want them tender but still with some bite — not mushy.
Pour in the soy sauce, oyster sauce, sugar, and white pepper all at once. Toss everything together vigorously so each mushroom gets coated in the glossy sauce mixture. The sugar will help everything caramelize and balance the salty funk of the sauces.
Kill the heat immediately and scatter the green onions over everything. Give it one final toss so the onions wilt slightly from the residual heat but stay bright green and crisp. Serve right away while the mushrooms are still sizzling.