Heat vegetable oil in a large skillet or wok over medium heat. Add minced garlic and sauté until fragrant, about 1 minute. Add shredded cabbage, julienned carrot, bean sprouts, and spring onions. Cook for 2-3 minutes until the vegetables are slightly softened but still crisp. If using, add cooked chicken, shrimp, or tofu. Stir in soy sauce, oyster sauce (if using), sesame oil, salt, and pepper. Cook for another 1-2 minutes. Remove from heat and let the filling cool.