Bring a large pot of water to a boiling and then cook the lamb leg for 2-3 minutes. Transfer out and clean with warm water.
Place the lamb in a clean pot, add green onion, ginger and all of the spices. Add enough water to cover (for regular pot, the water level should be at least 3 cm higher than the lamb). Optionally add white radishes cubes.
Bring to a boiling and then simmer for around 1 hour. Cover and let the lamb cool down completely in the soup.
Make the dipping sauces
Dry spice mixture: heat around 1 teaspoon of salt over slow fire for 1 minute or until slightly browned. Add 1 tablespoon of Chili pepper flakes and 1 tablespoon of ground cumin. Heat over slowest fire until aromatic. Transfer out to cool down.
Hot oil mixture: Mix 2 tablespoons of chili pepper flakes, 1/4 teaspoon of Sichuan peppercorn, 1 teaspoon of ground cumin, 2 minced garlic cloves and 1/2 teaspoon of salt in a small bowl. Heat 1/4 cup of oil in a small pot until almost smoky. Pour the hot oil into the mixture. Wait until slightly cooled. Then mix in sesame oil, light soy sauce, vinegar and minced coriander stem.
Transfer it out and drain for 1 hour or in fridge overnight. Finely slice the lamb. Lay a thin l layer of coriander in the serving plate and place the lamb slices one by one.