Set your oven to 350°F (175°C). Pat beef cubes dry and season with salt and pepper to ensure a perfect sear.
Heat oil in your Dutch oven and sear beef in batches until golden brown. This caramelization builds deep, rich flavor in the final dish.
Reduce heat to medium and stir in lemongrass, serrano peppers, ginger, garlic, lime leaves, curry paste, and turmeric. Cook 2–3 minutes until fragrant.
Add mushrooms and cook until slightly browned. Pour in coconut milk and water, then stir in palm sugar and fish sauce, adjusting seasoning to taste.
Return beef to the pot with onions and carrots. Cover and braise in the oven for 2½–3 hours until beef is fork-tender.
Discard lemongrass and lime leaves. Let the dish rest for 30 minutes to deepen the flavors. Serve with rice, fresh cilantro, lime wedges, and optional chili oil.