Heat the vegetable oil in a large pot over medium heat. Add the minced lemongrass, garlic, and ginger and cook for one minute until fragrant.
Add the pork tenderloin strips and cook for two to three minutes, stirring until the meat is no longer pink on the outside.
Stir in the sliced onion and julienned carrots. Cook for another two minutes to soften the vegetables slightly.
Pour in the chicken broth and fish sauce. Bring the mixture to a gentle simmer and cook for 10 minutes so the flavors blend and the pork finishes cooking.
While the stew simmers, cook the rice noodles in a separate pot according to package directions. Drain and divide them among four bowls.
Stir the lime juice into the stew. Ladle the hot stew over the noodles and top each bowl with cilantro, mint, basil, and green onions.