Preheat BBQ to 350°F (177°C) with a cast-iron rib roaster inside.
While on high, add olive oil, garlic, and sliced chili peppers to the roasting pan and lightly saute garlic until it begins to brown.
Rough chop mushrooms, zucchini, celery, and eggplant into similar sizes and add all vegetables to the roasting pan. Make sure to move vegetables around, coating them in oil to prevent sticking.
Mix in oyster sauce and soy sauce and roast at 350°F (177°C) for 30 minutes or until vegetables are tender and have started braising when the sauce reduces.