Slice the eggplant and onion into bite-sized pieces.
Add sesame oil to a pan and stir-fry the onion.
When the onion becomes translucent, add the eggplant and stir-fry together.
Add tuna and continue stir-frying.
Add doenjang (soybean paste) and water, then simmer.
When all ingredients are cooked through, finish the dish.