Peel the onion, cut it into cubes and add a little salt (maybe even steam it in the microwave for 1 minute so that it doesn`t taste so hot).
Peel the cucumber and wash the tomatoes. Also cut into small pieces and add to the onion. Season with salt, pepper and curry powder and add balsamic vinegar, a little ketchup, a little tomato paste and olive oil if you like. Finally add the tuna and a little sugar.
The salad should sit in the fridge for about 1 hour and it tastes best when it is taken out of the fridge about 10 minutes before serving.