Wash the cabbage thoroughly and slice it thinly.
Slice the onion into thin strips.
Heat some cooking oil in a pan and sauté the onion.
When the onion becomes translucent, add the cabbage and stir-fry together.
Add the tuna (drained of oil) and mix well.
Season with soy sauce and pepper.
Once all the ingredients are cooked, place them in a bowl.