Chop the scallion and separate white and green parts. Finely dice the tomatoes and set them aside in a bowl.
Preheat a medium size soup pot over medium heat, add oil. Saute white scallion parts with two pinches of salt until fragrant, about 8-10 seconds. Take care not to burn them.
Add tomatoes and 2 pinches of salt. Saute for 2 to 3 minutes or until the tomatoes turn quite soft in texture.
Add broth and cover with a lid. Lower the heat to medium-low. Bring it to a gentle simmer and cook about 8 to 10 minutes total to extract more flavor from the tomatoes.
In the meantime, whisk the eggs in a measuring cup with spout until you can see small bubbles form. In a separate small bowl, mix well arrowroot and 2 tbsp water to make a slurry. If you are on keto, skip the slurry step.
Remove the lid and taste test the soup to adjust salt seasoning. Aim on the less salty side as the soup will taste saltier once it comes to room temperature.
Bring the soup to hot simmering and almost to a gentle boiling state. Stir the starch mixture one more time then slowly drizzle it into the broth, stir for about 30 seconds.
With one hand slowly drizzle in the eggs from higher up and the other hand quickly stir the soup constantly with chopsticks in the same circular motion to form fine egg flower ribbons.
Off heat, drizzle with toasted sesame oil and garnish with cilantro, if using, and green scallion parts. Serve hot or warm.