Preparing the base - Cook prahok in oil over low heat until the moisture evaporates, add lemongrass paste and continue stirring until paste is firm and peels away from the pot.
Cook beef - Increase heat to medium, add meat, stir and continue cooking until meat is completely seared. Add 1/2 cup beef broth and bring to boil, then cover and reduce heat; let simmer until the beef is tender. Stir occasionally. You may need to add more broth and simmer longer to get the meat done the way you like it.
Prepare water spinach - While the beef is cooking, trim and discard the old stems from water spinach. Remove most of the leaves, keep only a few at the very top. Wash and strain, then use a clever to smash the stems and cut into two inch lengths. Separate the stems and the tops of the water spinach; put cut stems into a large bowl of cold water. Set aside.Note - The cold water bath allows the water spinach to release its sap.
Add broth and tamarind - After the beef is tender, increase heat to high, add remaining broth and bring to boil. Add tamarind.Note - It is important to add tamarind prior to adding the water spinach, which allows the vegetables to stay crunchy.
Add vegetable - Add water spinach stems and stir, turn off the heat and add the water spinach tops, chiles, and hot basil. Salt to taste. Do not cover the pot.Note - Covering the pot while it's still hot will cause the green vegetables to turn brown.
Best served with steam rice and fish sauce with birds eye chili. *Variations and Substitutions:
Prahok is Cambodian fermented fish paste. The best substitute for prahok is fish sauce or anchovy paste.
Variations of different protein used for this soup include: fatty beef trimmings, cow stomach and tripe, pork spare ribs, and freshwater fish or shrimp. My favorite is with fish, specifically grilled fish that has been deboned and then added to the soup while the broth is boiling.
Ripe and concentrated tamarind paste adds sour and a fruity notes to the soup. In Cambodia, acidity for the soup comes from many source including: young tamarind leaves, tamarind flowers, green tamarind fruit, ambarella (kuntout) leaves , feroniella lucida (krawsankg) fruit, schleichera (pongro) berries, and even green mango. Each of these sour leaves, flowers, fruit and berries brings out their own unique flavor in the soup. The decision of which sour ingredient to use most often depends on seasonal availability. Many times the soup is made just for the taste of certain kind of sourness before its season is over. Since ripe tamarind can be preserved year round, it is the most common recipe for the soup, especially when cooking with beef.
Water spinach is wild, abundant and available year round in Cambodia. During the monsoon season, water spinach is most tender and luscious, it grows mostly under flood water in the rice fields or near a lake. During the dry season it grows like ground cover with short and tender tops packed with sap and strong flavor. Thus, during the dry season most people would rather make the soup with other vegetables such as kai choy, or mustard greens. You can also add other variety of vegetable into the soup along with the water spinach. They include pearl eggplants, Thai eggplants, jalapenos, and celery. If you have to use vegetables other than water spinach, reduce the prahok to one tablespoon. For some reason water spinach sucks up the prahok flavor, leaving the soup less bold.
Hot basil (M'reas prouv) has similar aroma as oregano but not as strong, thus to substitute hot basil with oregano use about 1/4 to the quantity calls out in the recipe. A variation to the soup is to use curry leaves (Sloek kontroap), by roasting them over a hot charcoal if you happen to have BBQ while cooking the soup. For convenience, roast the leaves in oven at 375ºF for 10 minutes. Use your hands to crumble the leaves while adding to the soup.