If you are using fresh tomatoes, peel, deseed and cut them into 2.5cm(1in) cubes. If you are using tinned tomatoes, chop them into small chunks. Carefully cut the beancurd into small chunks as well. In a small bowl, lightly beat the egg and then combine with the sesame oil.
Put the stock into a wok and bring it to simmering point. Add the sugar, salt and soy sauce, and stir to mix well. Then add the beancurd and simmer for 4 Minutes.
Next add the tomatoes and simmer for 2 Minutes. Stir in the spring onions.
Now add the egg mixture in a very slow, thin stream. Using a chopstick or fork, pull the egg gently into strands. (I find that stirring the egg in a figure of eight works quite well.)
Turn into a soup tureen or individual bowls, garnish with the spring onion tops and serve at once.