Plunge the tomatoes in boiling water for a moment, skin them and cut them in half. Press each half in your hand to squeeze out the seeds and excess juice, then cut the tomato flesh into small dice. Chop the onion.
Put half the butter - 30 g (1 oz) - into a sauté pan and sweat the onion gently for 5 minutes without browning. Add the tomatoes, the two unpeeled cl