First, clean the bull's testes and tendons thoroughly in salted water. Cut into large chunks. Parboil twice in fresh water to remove any gamey taste, about 5 minutes each time.
In a large pot, combine the parboiled meat, tendons, water, beef stock, bruised lemongrass stalks, and sibot herbs. Bring everything to a boil on high heat. Once boiling, lower the heat and cover. Let it simmer gently for 3.5 hours until the meat becomes very tender.
While waiting, prepare your aromatics. Slice your ginger thinly, chop the onions, mince the garlic, and cut the spring onions, keeping the white and green parts separate.
Once the meat is tender, remove it from the broth and cut into bite-sized pieces. Strain the broth and throw away the lemongrass and herbs. Keep the broth.
Get a clean pot and heat some oil over medium heat. Cook the garlic first until it turns light golden. Add the onions and white parts of spring onions, cooking until soft. Add the ginger and cook until you can smell its aroma.
Put the sliced meat and tendons back in. Add the chilies and cook everything together for 2 minutes.
Pour in your saved broth. Let it come to a boil, then reduce heat and simmer for 30 minutes so the flavors can come together.
Add fish sauce to taste and plenty of freshly ground black pepper. Give it 5 more minutes to simmer.
Serve hot in bowls, topped with the green parts of spring onions you saved earlier.
This soup tastes best piping hot and is perfect as a late-night dish or early morning pick-me-up.