Step 1

Preheat oven to 180°C. Grease 2 baking trays and line with baking paper.

Step 2

Spread tomatoes, onion and garlic evenly across prepared trays, then season with 1 tbs salt flakes and 1 tsp freshly ground black pepper. Divide oil evenly between trays and toss to combine. Roast, swapping trays halfway, for 45 minutes or until tomatoes are blistered and onion has softened. Set aside to cool for 30 minutes, then stir through basil.

Step 3

Pass tomato mixture through a mouli or nylon sieve into a bowl and extract as much juice as possible. Reserve any remaining seeds and skins to make tomato conserva.

Step 4

Sterilise bottles and lids and pour passata into hot jars. 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.

Step 5

Line the base of a large stockpot (it should be deep enough to submerge the jars under water) with a tea towel and, working in batches if necessary, add jars, making sure they don’t touch the sides of the pot or one another.

Step 6

Roughly matching the water temperature to the temperature of the jars (this prevents the jars breaking when the water is added), pour in enough water to completely cover the jars. Slowly bring to the boil over medium heat. Boil for 40 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. 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.

Step 7

The next day, the jar lids should be concave, confirming they are vacuumsealed. If lids are not concave, passata should be stored chilled and used within 2 weeks. Sealed jars can be stored in a cool, dark place for up to 1 year.