Heat a large heavy-bottomed pot over medium heat and add a tablespoon of oil.
Add diced onion to the pot and sauté until translucent, about 5 minutes, until it starts to smell sweet and becomes slightly golden.
Toss in minced garlic, grated ginger, and chili flakes, cooking for about 1 minute until fragrant and the mixture begins to bubble softly.
Stir in chopped tomatoes and cook until they soften and release their juices, around 8 minutes, stirring occasionally.
Add the shredded cooked chicken and pour in the chicken broth, stirring to combine. Bring the mixture to a gentle simmer.
Allow the soup to simmer gently for 15-20 minutes, skimming off any foam or excess fat that rises to the surface.
Stir in fish sauce and squeeze in fresh lemon juice, then taste and adjust seasoning with salt, pepper, or more chili flakes if desired.
Let the soup cook for an additional 5 minutes to meld the flavors, then give it a final stir.
Serve the hot, vibrant broth in bowls, garnished with fresh herbs or sliced chili if you like extra heat.