In a large bowl, toss the beef cubes with the Chinese five-spice powder and a pinch of salt until evenly coated.

Heat the vegetable oil in a heavy-bottomed pot or Dutch oven over medium-high heat. Sear the beef in batches until browned on all sides, about 3-4 minutes per batch, then remove and set aside.

In the same pot, add the onion, garlic, ginger, and lemongrass stalks. Sauté for 3-5 minutes until the aromatics are fragrant and the onions are softened.

Stir in the tomato paste and cook for 1 minute to deepen the color, scraping up any browned bits from the bottom of the pot.

Return the beef to the pot and pour in the beef broth, fish sauce, and brown sugar. Bring the mixture to a boil, then reduce heat to low, cover, and simmer for 1.5 hours.

Add the carrot chunks to the pot and continue to simmer uncovered for another 30-40 minutes, or until the beef is fork-tender and the sauce has thickened slightly.

While the stew finishes, cook the rice noodles according to the package instructions, then drain and rinse with cold water to prevent sticking.

To serve, divide the cooked noodles among bowls and ladle the hot beef stew and carrots over the top.

Garnish generously with fresh cilantro, Thai basil, and a squeeze of lime juice for a burst of acidity.