In a large pot, heat 1 tablespoon of olive oil over medium heat. Add sliced celery, diced onion, minced garlic, and minced ginger. Sauté for about 5 minutes until the vegetables are tender and fragrant.
Add the sliced carrots to the pot and continue to sauté for another 3-4 minutes to enhance the flavor of the broth.
Pour in 6 cups of chicken broth and stir in 3 tablespoons of fish sauce. Add lemongrass, star anise pods, and black peppercorns. Bring to a gentle boil.
Add the trimmed chicken thighs and 1 tablespoon of Chinese five-spice. Reduce heat and simmer for about 20-25 minutes, or until the chicken is cooked through.
Remove the chicken from the pot, let it cool slightly, then shred it into bite-sized pieces and return it to the pot.
Prepare the rice noodles according to package instructions, then drain and set aside.
Just before serving, stir in the juice of half a lime and sriracha to taste. Adjust seasoning as needed.
Divide the cooked rice noodles among bowls, ladle the hot broth with chicken and vegetables over the noodles, and garnish with sliced mint, Thai basil, and cilantro.
