Step 1: If using chicken or tofu, slice it thinly and marinate with 1 tablespoon soy sauce and 1 teaspoon cornstarch for at least 10 minutes while you prep other ingredients.
Step 2: Core and thinly slice the cabbage into ribbons. Rinse well and drain thoroughly.
Step 3: Mince garlic and grate ginger. Slice green onions for garnish.
Step 4: In a small bowl, whisk together the stir-fry sauce: 3 tablespoons soy sauce, rice vinegar, brown sugar, toasted sesame oil, and red pepper flakes (if using). Set aside.
Step 5: Heat 1 tablespoon of cooking oil in a large wok or skillet over high heat until shimmering. Add the marinated chicken (or shrimp) in a single layer and cook for 2-3 minutes per side until browned and mostly cooked through. Remove from the pan and set aside.
Step 6: Add the remaining 1 tablespoon of cooking oil to the hot pan. Add the minced garlic and grated ginger. Stir-fry for 30 seconds until fragrant, being careful not to burn.
Step 7: Add the sliced cabbage to the pan. Stir-fry vigorously for 3-5 minutes, tossing constantly, until the cabbage begins to soften and turn vibrant green but still retains a slight crispness.
Step 8: Return the cooked chicken (or shrimp, or tofu) to the pan. Give the stir-fry sauce another quick whisk, then pour it over the ingredients in the pan.
Step 9: Toss everything together for 1-2 minutes, allowing the sauce to thicken slightly and coat all the ingredients evenly.
Step 10: Remove from heat. Taste and adjust seasoning if needed. Garnish with sliced green onions and sesame seeds before serving hot.