Take the seeds out of the bitter melon and slice thinly. Massage with a little bit of salt in a pan. Add enough water to cover, bring to a boil and cook for 30 seconds. Drain and immediately transfer to a bowl of ice water to crisp up the slices.
Drain and squeeze out the bitter melon very well and add to the bowl from Step 1. Add the thinly sliced onion and drained tuna also and mix well. You can add some tomato too if you like.
If it's too salty, add a bit of lemon juice.
Story Behind this Recipe
One Cookpad user called Setsubunhijiki had uploaded a recipe for "Bitter Melon and Crispy Pork With Spicy Mayonnaise" which was really delicious so I rearranged it a bit and added my own touches. I used sweet chili sauce plus mayonnaise to make it simple. The ponzu sauce really brings the flavors together.
Just use a pinch of salt to massage the bitter melon, or else the dish will be too salty. Don't boil the bitter melon for too long. Put the onions in a bowl of water to get rid of the harsh flavor.