1. Use three chiles for a salsa of medium heat, up to size if you like it hot. To peel the chiles, spear them on a long-handled metal skewer and roast them over the flame of a gas burner until the skins blister and darken. Do not let the flash burn. Alternatively, dry-fry them in a griddle until the skins are scorched.
2. Place the roasted chiles in a strong plastic bag and tie the top of the bag to keep the steam in. Set aside for about 20 minutes.
3. Meanwhile, chop the onion finely and put it in a bowl with the line rind and juice. The line juice will soften the onion.
4. Remove the chiles from the bag and peel off the skins. Cut off the stalks, then slit the chiles and scrape out the seeds with a sharp knife. Chop the flesh roughly and set aside.
5. Cut a small cross in the bottom of each tomato. Place the tomatoes in a heatproof bowl and pour in boiling water to cover.
6. Leave the tomatoes in the water for 3 minutes, the lift them out using a slotted spoon and plunge them into a bowl of cold water. Drain. The skins will have begun to peel back from the crosses. Remove the skins completely.
7. Dice the peeled tomatoes and put them in a bowl. Add the chopped onion which should have softened, together with the lime mixture. Chop the cilantro finely.
8. Add the cilantro the salsa, with chiles and the sugar. Mix gently until the sugar has dissolved and all the ingredients are coated in lime juice. Cover and chill for 2-3 hours to let flavors blend. The salsa will keep for 3-4 days in the refrigerator. Garnish with the strips of lime zest just before serving.
