01 - Season beef cubes generously with salt and pepper on all sides.
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, approximately 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, being careful not to burn the garlic.
04 - Stir in fish sauce and brown sugar; cook for 1 minute to dissolve the sugar and allow flavors to meld.
05 - Return the beef and any accumulated juices to the pot. Add coconut milk, beef broth, and carrots. Bring to a gentle simmer, stirring to combine.
06 - Reduce heat to low, cover tightly, and braise for 1 hour. Maintain a gentle simmer throughout.
07 - Add quartered mushrooms to the pot, stir gently, and continue braising covered for another 1 to 1.5 hours until beef is fork-tender.
08 - Remove and discard 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, garnished generously with fresh cilantro, sliced scallions, and additional lime wedges on the side.