Cut the kimchi into bite-sized pieces.
Slice the onion thinly.
Heat sesame oil in a pan and sauté the onion.
When the onion becomes translucent, add the kimchi and stir-fry.
Drain the oil from the tuna using a strainer.
Add the drained tuna to the stir-fried kimchi and mix together.
Taste and adjust seasoning with salt or pepper if needed.