Method

• 1.
  Preheat oven to 200°C/180°C fan-forced. Place tomatoes, cut side up, in a single layer on 2 large baking trays. Sprinkle with onion, herbs, garlic, chilli (if using) and sugar. Drizzle with oil. Season very well with salt flakes and freshly ground black pepper.
• 2.
  Bake for 55 minutes-1 hour until tomatoes are caramelised and onion is cooked. Set aside on trays to cool before placing in a food processor. Whiz until smooth, then strain the mixture through a sieve into a large bowl. Discard any seeds and skin, as these will make the sauce bitter.
• 3.
  Divide sauce among 2 sterilised 3-cup (750ml) preserving jars with screw-top lids. Push basil leaves into mixture. Tap jars on a work surface and run a clean knife around the inside to remove any air bubbles. Wipe the rim of the jars with paper towel and seal.
• 4.
  Line the base of a large stockpot (it should be deep enough to submerge the jars underwater; check this before you start bottling) with a tea towel and, working in batches if necessary, add jars, making sure they don’t touch the sides of the pot or each other.
• 5.
  Roughly matching the water temperature to the temperature of the jars (this prevents jars breaking), pour in enough water to completely cover the jars. Slowly bring to the boil over medium heat. Boil for 1 hour 15 minutes, adding more boiling water when necessary so the jars are completely submerged at all times, or until the jar lids are puffed up and convex.
• 6.
  Using a heatproof jug, carefully remove enough water from the pot to allow you to carefully remove jars from water using oven gloves or a thick tea towel. Set aside at room temperature overnight. The next day, the jar lids should be concave, confirming they are vacuum-sealed. If lids are not concave, sauce should be stored chilled and used within 2 weeks. Sealed jars can be stored in a cool, dark place for up to 1 year.