01 - Season beef cubes evenly with kosher salt and black 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, approximately 3-4 minutes per batch. 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, stirring occasionally.
04 - Stir in fish sauce and brown sugar, cooking for 1 minute to allow the sugar to dissolve and caramelize slightly.
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 with a tight-fitting lid, and braise for 1 hour. The liquid should maintain a gentle simmer throughout.
07 - Add the quartered mushrooms and stir gently to incorporate. Continue to braise covered for another 1 to 1.5 hours, until beef is fork-tender and easily pulls apart.
08 - Remove and discard the lemongrass stalks. Stir in the 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 fresh cilantro, sliced scallions, and additional lime wedges on the side.