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In a large shallow pan, fry the onion and garlic in the olive oil on a low heat for 8 mins; do not brown. Roughly chop the tomatoes and add along with the sugar, vinegar, 750ml water and seasoning. Bring to the boil and simmer for 35 mins, stirring from time to time.
Whizz with a hand-held blender until smooth. For an ultra-smooth, dinner party-style soup, you can pass the liquid through a fine sieve, but this is quite fiddly and your blender will give perfectly good results.
For the gremolata, mix together the lemon zest, juice, olive oil, garlic and parsley, then serve on top of the soup.