Prep & Marinate: Cut beef shank into large chunks. Combine with soybean paste, chopped lemongrass, garlic, shallots, fish sauce, and spice mix. Toss well and marinate for at least 1 hour, or ideally overnight.
Sear: Heat oil in a heavy pan. Add beef in a single layer and sear until browned on both sides. Sauté extra shallots and garlic at the end, then transfer to a large pot.
Build Broth: Add the star anise and cloves in a tea bag. Stir in tomato paste, pour in coconut water, and add enough plain water to cover the beef. Season lightly with chicken bouillon and fish sauce.
Aromatics: Add smashed lemongrass stalks, ginger, and rock sugar. Bring to a boil, then lower to a gentle simmer.
Slow Simmer: Cook uncovered for 1.5–2 hours until the beef is tender but still intact. Adjust seasoning with more fish sauce or salt as needed.
Carrots: Add sliced carrots for the final 15 minutes of cooking.
Serve: Ladle over rice noodles (Hủ Tiếu Bò Kho) or serve with a crispy baguette. Garnish with green onion, white onion, cilantro, Thai basil, and a squeeze of lime.