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Heat the oil in a saucepan then add the vegetables and cook them over a medium-high heat for 10-15 mins to brown – if they burn at the edges, then all the better. Stir in the ketchup and dried mushrooms and cook together until everything becomes sticky, then splash in the vinegar. Stir in the stock and season with a pinch of salt, if you like. Bring to the boil, then simmer gently for 20 mins.
Using a hand blender, blitz to create a thin soup-like consistency, then pass through a sieve into another saucepan and bring to the simmer. Slake the cornflour with a splash of water, then pour into the liquid and continue to cook until thickened. Serve straightaway, chill, or freeze for reheating at a later date. Can be frozen for up to three months in an airtight container.