Preserve us: making the most of tomatoes

Makes about 1 litre

250ml olive oil
1 large onion, peeled and roughly chopped
2kg ripe tomatoes, cored and peeled (peeling is optional)
1 whole head of garlic, peeled and separated into cloves
1 rounded tsp sea salt
freshly ground black pepper
zest of 1 small lemon (optional)
1 tsp pale golden caster sugar
1 large handful basil

1. Heat a little of the olive oil in a large, heavy-bottomed pan and fry the onion until golden. Add the remaining ingredients and cook slowly for 2-3 hours, stirring occasionally. You don’t need to put a lid on as long as you keep the heat very low.
2. If you want a very smooth sauce, you’ll need to peel the tomatoes or, if you prefer, blend the whole lot through together at the end (although this does decrease the brightness of the sauce). The sauce should be thick and jammy.
3. While the sauce is still hot, pour it into sterilised rubber-sealed jars and allow to cool. Keep in the fridge until needed.
4. If you want the sauce to last indefinitely, bring a pan of water to the boil and wrap the hot filled jar in newspaper. Fold up some more newspaper and place it under the jar to prevent it cracking. Carefully lower the jar and newspaper into the pan, making sure that the jar is immersed. Bring the water to a gentle simmer and leave for about 45 minutes.
5. Allow the jar to cool in the water before attempting to lift it out. Once you have sealed the lid, it should be difficult to lift it open, which is proof that you have created the necessary vacuum. Don’t be tempted, however, to open the jar at this stage. Your sauce is now ready to store.