Shred the cabbage.
Slice the onion thinly.
Add cooking oil to a pan and stir-fry the onion.
Add the cabbage and stir-fry together.
Drain the oil from the canned tuna and add the tuna to the pan. Stir-fry.
Season with soy sauce and black pepper.
Once all ingredients are cooked, turn off the heat and finish.