Place the chicken breasts in a saucepan and cover with water. Place over a high heat until just boiling, then reduce the heat and allow to gently simmer for 10-15 minutes depending on how thick the chicken breasts are. You will find that some white bits float to the top. Use a spoon to skim off any white bits that float to the top as you will use the poaching liquor for your soup.
While the chicken is poaching, spray another saucepan with low calorie cooking spray and sauté the onions over a medium heat for 5 minutes until soft but not coloured.
Add the garlic and ginger and cook for a further minute.
Remove the chicken breasts from the water, reserving the liquor and shred using 2 forks.
Make up the stock pot with 1 litre of the reserved poaching liquor and add to the pan.
Add the sweetcorn, chicken, soy sauce and rice vinegar, bring to a simmer and cook for 10 minutes.
Mix the cornflour with a tablespoon or 2 of cold water and stir into the soup to thicken (as an alternative you can use 1/4-1/2 tsp of xanthan gum to thicken).
Making sure the soup is hot and simmering, slowly drizzle the beaten egg onto the top of the soup. As it hits the hot soup, it will cook. Using a spoon, gently swirl the egg across the soup as it cooks, to produce thin trails of egg.
Ladle into warmed bowls and serve garnished with spring onions.