Clean and wash the bull organ and oxtail, then place in a large pot.
Pour enough water to cover the meat. Bring to a boil and let it boil for 5 to 10 minutes.
Remove from heat and discard water, wash off all scum from the meat, and return to the pot. Pour fresh water to cover meat at about 2 inches.
Add in the whole garlic, sliced ginger, quartered onion, peppercorns, and sibot mix herb.
Bring to a boil and simmer for 4 to 6 hours at a low to moderate heat.
Add more water as necessary.
When done, remove pot from the heat and separate the broth from the meat. Using a sieve, strain off all solid residue from the broth.
Discard residue and put aside the broth.
Slice the bull organ and oxtail into thin slices. Discard the oxtail bones.
In a saucepan, sauté the chopped garlic, ginger, and onion.
Add in the sliced meats and continue to stir-fry for 2 to 3 minutes.
Add in the fish sauce and stir-fry for another 1 to 2 minutes.
Add in the reserved broth, bring to a boil, and simmer for 10 to 15 minutes.
Season with salt and pepper to taste.
Add in the chopped spring onion and cook for another minute.
Serve