Wash the cabbage thoroughly and slice it thinly.
Slice the onion thinly.
Slice the carrot thinly.
Heat cooking oil in a pan and stir-fry the onion and carrot.
Add the cabbage and stir-fry together.
Remove oil from tuna and add it to the pan.
Season with soy sauce and pepper.
Place the stir-fried mixture over rice.
Serve with mayonnaise and parsley.