Heat the vegetable oil in a large Dutch oven or heavy-bottomed pot over medium-high heat. Add the beef chunks in batches, searing until browned on all sides, about 5 to 7 minutes per batch. Transfer the browned beef to a plate and set aside.

In the same pot, add the chopped onion, minced garlic, and sliced ginger. Sauté for 3 to 4 minutes until the onions are softened and fragrant, scraping up any browned bits from the bottom of the pot.

Stir in the tomato paste and cook for 1 minute until it darkens slightly. Return the beef and its resting juices to the pot, then add the bruised lemongrass, star anise, fish sauce, brown sugar, and beef broth.

Bring the liquid to a rolling boil, then reduce the heat to low. Cover the pot and let it simmer gently for 1.5 hours, or until the beef is beginning to become tender.

Add the chopped carrots to the stew, cover again, and simmer for an additional 30 minutes until both the beef and carrots are fork-tender. Remove and discard the tough lemongrass stalks and star anise before serving.

While the carrots are finishing, bring a separate pot of water to a boil and cook the dried rice noodles according to the package directions. Drain and rinse the noodles briefly under warm water to prevent them from sticking together.

To serve, divide the cooked rice noodles among large, deep bowls. Ladle the hot, fragrant lemongrass beef stew generously over the noodles, ensuring an even mix of beef and carrots, and top with freshly chopped cilantro.