In a large, microwavable bowl, combine the chopped tomatoes, white wine vinegar, and sea salt.
Cover loosely with a microwave-safe plate or lid.
Microwave at full power (based on 800W) for 10–15 minutes, stirring every 5 minutes, until the tomatoes are completely soft and broken down.
Carefully remove the bowl from the microwave.
Add the caster sugar and tomato purée, then use a hand-held immersion blender to blitz the mixture to a smooth consistency.
Return the bowl to the microwave, uncovered.
Cook for a further 25–30 minutes, stirring every 5 minutes, until the jam has thickened significantly. The long cooking time is essential to reduce the water from the tomatoes.
To test if it's ready, place a small spoonful of jam onto a chilled plate. If it doesn't run when you tilt the plate, it's done.
Taste the jam and add more salt or sugar if you feel it's needed. Allow it to cool slightly.
While still warm, carefully pour the jam into a 600ml (20.3 fl oz) sterilised jar (or two smaller jars). Seal immediately. The jam will keep for up to a month in the fridge once opened.
Stovetop Method:* If you don't have a microwave, combine all in a heavy-based saucepan.
Cover and cook for 5 minutes.
Remove the lid and continue to cook, stirring frequently, until thick and it passes the chilled plate test.
Blend with a hand-held blender for a smoother texture.
Keep a small plate in the freezer before you start. To test if the jam is set, drop a small spoonful onto the cold plate, wait 30 seconds, then push it with your finger. If it wrinkles, it's ready.
Add a pinch of chilli flakes, a sprig of thyme, or a star anise during the initial cooking stage for a delicious flavour twist.
