Separate the cabbage leaves and wash them thoroughly.
Blanch the cabbage leaves in boiling water for 1–2 minutes, then let them cool.
Drain the oil from the tuna can and place the tuna in a bowl.
Mix tuna with mayonnaise, chopped onion, salt, and pepper.
Place the tuna filling on a cabbage leaf and roll it up tightly.
Slice into bite-sized pieces and arrange on a plate.