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Fry the veg and herbs in the butter for 10-12 mins until the vegetables start to brown. Scatter over the sugar and continue to cook until sticky and caramelised.
Stir in the flour until sandy, then add the Marmite, if using, tomato purée and vinegar. Pour over the stock, then simmer everything together until you have a thickened sauce.
Sieve, then add soy sauce to season and colour. Use straightaway or cool and freeze.
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