Boil the beef tail until tender.
Prepare a pound of the ingredients: 3 inches ginger, 4-5 cloves garlic, 2 tablespoons black pepper powder, salt and seasoning to taste, 2 bunches lemongrass, 3-4 large red chillies (optional), 2 large red onions, 2 inches galangal, 2 inches fresh turmeric, 2 inches belacan, 10 pieces serai cut into sections, 1 bunch kaffir lime leaves, 2 tablespoons tamarind paste, and 4-5 large red chillies for pounding.
Pound the prepared ingredients together with 2 tablespoons salted soy sauce.
Heat oil in a wok or pan and sauté sliced serai and a few kaffir lime leaves until fragrant.
Add the pounded mixture and fry until the oil separates from the spices.
Stir-fry the spice paste until aromatic, then add enough water.
Simmer for 20-30 minutes, then add extra kaffir lime leaves in one bunch and a little black pepper powder.
Continue cooking until well integrated with the beef tail.
If there is not enough liquid, add more water to ensure it thickens.
Season with salt and seasoning to taste.
Serve the asam pedas beef tail with steamed white rice for Iftar.