01 - Season beef cubes evenly with salt and pepper, coating all surfaces.
02 - Heat vegetable oil in a large Dutch oven over medium-high heat. Working in batches to avoid overcrowding, sear the beef cubes until browned on all sides, about 3-4 minutes per batch. Remove browned beef and set aside.
03 - In the same pot, add onion, lemongrass, garlic, ginger, and chilies if using. Sauté for 3-4 minutes until fragrant and onions are softened, stirring occasionally.
04 - Stir in fish sauce and brown sugar. Cook for 1 minute, allowing the sugar to dissolve and the mixture to become aromatic.
05 - Return the seared beef to the pot. Add coconut milk, beef broth, and carrots. Bring the mixture to a gentle simmer, stirring to combine all ingredients.
06 - Reduce heat to low, cover the pot, and braise for 1 hour. Maintain a gentle simmer throughout.
07 - Add quartered mushrooms, stir gently to incorporate, and continue braising covered for another 1 to 1.5 hours, until beef is very tender and easily pierced with a fork.
08 - Remove and discard the lemongrass stalks. Stir in lime zest and juice. Taste and adjust seasoning with additional salt or fish sauce if needed.
09 - Serve hot over steamed jasmine rice or rice noodles. Garnish generously with chopped fresh cilantro, sliced scallions, and additional lime wedges on the side.