Preparation / Directions:
1. Soak tamarind pulp in the hot water until it is soft.
2. Heat oil in a medium saucepan over medium heat. Add ginger, green onions, lemongrass, and hot pepper flakes and cook just until fragrant, about 30 seconds.
3. Strain tamarind water through a fine-mesh strainer into the same saucepan. With a wooden spoon, push pulp through strainer into the sauce pan, straining out and discarding seeds and fibers. Add fish sauce to pan. Simmer 5 minutes and taste for seasoning--it should be tart and slightly hot. Adjust seasonings if necessary. (May be prepared to this point up to an hour ahead.)
4. Have soup at a simmer. Add catfish slices and cook just until opaque white. Serve immediately, garnished with cucumber slices.