01 - Season beef cubes evenly with kosher salt and black pepper, ensuring all pieces are coated.
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. Remove beef and set aside on a plate.
03 - In the same pot, add onion, lemongrass, garlic, ginger, and red chilies if using. Sauté for 3–4 minutes until fragrant and onions are softened, scraping up any browned bits from the bottom.
04 - Stir in fish sauce and brown sugar. Cook for 1 minute to allow the sugar to dissolve and flavors to meld.
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.
06 - Reduce heat to low, cover tightly, and braise for 1 hour. Check occasionally to maintain a gentle simmer.
07 - Add quartered mushrooms and stir gently to incorporate. Continue braising covered for another 1–1.5 hours until beef is fork-tender and mushrooms are cooked through.
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 in bowls, generously garnished with chopped cilantro, sliced scallions, and additional lime wedges on the side.