Wash the cabbage, onion, and zucchini thoroughly and cut them into bite-sized pieces.
Heat cooking oil in a pan over medium heat.
Add the onion first and stir-fry for 1 minute.
Add the cabbage and zucchini and stir-fry for 2~3 minutes.
Season with salt and pepper, and stir-fry until all the vegetables are cooked through.
Turn off the heat, transfer to a plate, and serve.