Begin by preparing all your ingredients. Chop the cabbage, slice the red bell pepper, julienne the carrots, mince the garlic and ginger, and slice the green onions.
Heat the vegetable oil in a large skillet or wok over medium-high heat. Add the minced garlic and ginger, cooking for about 30 seconds until fragrant.
Add the chopped cabbage and stir-fry for about 5 minutes, until it begins to wilt.
Next, add the sliced red bell pepper and julienned carrots. Continue to stir-fry for an additional 5 minutes.
If using protein, add it at this stage and stir well to combine all ingredients.
Pour in the soy sauce, rice vinegar, sesame oil, and red pepper flakes (if using). Stir everything together and let it cook for another 2-3 minutes. Season with salt and pepper to taste.
Finally, toss in the sliced green onions just before serving.