Place your chicken breast in a pan of boiling water. Leave to simmer for 20 minutes and then remove and set aside.
Meanwhile, place another saucepan on medium heat, drizzle with a bit of oil and add in your chopped tomatoes, onion, ginger, and garlic. Give everything a good mix and fry for 3 minutes.
Add in your teriyaki sauce, sesame oil, fish sauce, sugar, and vegetable stock with water. Simmer for 5 minutes and then in a small bowl, combine your cornstarch with 1 tablespoon of warm water. Give it a good mix and then pour it into your soup. It should thicken up the soup slightly giving it a glossy look. Leave to simmer while you shred your chicken.
Get your poached chicken and shred it using a couple of forks. Once shredded, add to your teriyaki soup.
Get your whisked eggs and pour them into your soup. Give everything a mix, taste, and adjust to your liking.
Prepare your udon noodles according to package instructions.
Place your udon noodles at the base of your bowl and top with the chicken teriyaki soup. Garnish with spring onions and chilli oil, enjoy!