Pour couscous into a bowl and pour boiling water over it at around 1 cm above the surface of the groats. Cover the dish with a plate and wait approx. 5-10 minutes, until the groats soak up the water.
Drain the tuna from oil and crush with a fork. Finely chop the onion and chives, and dice the pepper. Mix the couscous, tuna, onions, chives (leave a little to sprinkle the meal) peppers and corn. Season with salt and pepper. Leave the salad in the fridge for an hour. Cut off the tips of the tomatoes and hollow out the center. Fill them with salad. Sprinkle with chives.