Preheat the oven to 180°C/Gas Mark 4.
For the tomato purée, halve the tomatoes and place them skin side down on a baking tray. Sprinkle with the salt, garlic and olive oil. Roast for an hour or so, on the bottom shelf of the oven, then remove. Rub through a sieve, or pass through a mouli or food mill, to remove the skins and pips – you’ll end up with about 300ml of intensely flavoured purée.
Meanwhile, put the beetroot into a baking dish and trickle with a little oil. Roast, above the tomatoes, for 1–1½ hours (longer if necessary) until the skins are blistered, blackened and loosened. Leave to cool a little before peeling. You’ll find the skins will slide off easily. Coarsely grate the beetroot (a food processor makes this job easy).
Put the sugar, vinegars, onion and horseradish into a large saucepan, bring to the boil and cook for 5 minutes. Stir in the tomato purée and cook for a couple more minutes. Finally, add the grated beetroot and cook for about 10 minutes, until thickened. Transfer to sterilised jars and seal with vinegar-proof lids.
Use within a year. Refrigerate once opened.
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