Wash and chop the vegetables. Dice the onion, slice the carrots and celery, and mince the garlic. Grate the fresh ginger.
In a large pot, heat the vegetable oil over medium heat. Add the diced onion and sauté for 5 minutes until translucent. Stir in the minced garlic and grated ginger, cooking for an additional 1-2 minutes.
Add the bite-sized pieces of chicken thighs to the pot. Stir well and cook for about 5 minutes until the chicken is slightly browned.
Pour in the chicken broth and water, stirring to combine. Add the sliced carrots and celery, and season with soy sauce, salt, black pepper, and red pepper flakes. Bring to a boil.
Once boiling, reduce the heat to low and let the soup simmer for 15 minutes.
Add the egg noodles and cook according to package instructions, usually about 5-7 minutes until al dente. Taste the soup and adjust seasoning if needed. Serve hot, garnished with sliced green onions and fresh cilantro.