Place the beef shank in a pot. Pour in enough water to cover. Bring to the boil. Boil the beef for 10 minutes. Strain, rinse and strain again. Throw out the cooking water. This pre-boiling prevents the formation of scum when the meat is transferred to the slow cooker.
Place the rinsed meat in the slow cooker. Pour in as much water as the cooker can handle. Add the lemongrass stalks. Season with about a tablespoonful of fish sauce. Cook on HIGH for two hours. Continue cooking on low for another eight hours.
When the beef is nearing its 10th hour of cooking, peel the green papaya and scrape off the seeds. Cut the flesh into wedges. Add to the beef. Throw in the chilies too. Taste the broth and add more fish sauce, if needed. Cook for 45 minutes.
Pour the tamarind extract to the pot. Sprinkle in the sugar.
Add the spinach leaves (stir if the cooker is not packed too much). Cook for another 10 minutes.
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