Bring water to boil over high heat in a medium-sized saucepan. Add the lemon grass, galangal, kaffir lime leaf, fish sauce and tamarind paste (break apart the tamarind paste with your fingers as you add it). Add the shrimp, bring to a boil and cook 3 minutes. Add the onion, nam prik pao and straw mushrooms. Boil for another 7 minutes until the shrimp is cooked through. Add the chile peppers and tomatoes. Turn off the heat. Add the lime juice. Taste to adjust the seasoning, adding fish sauce to taste. Garnish with cilatantro, roasted whole chili peppers and a splash of coconut mik if desired and serve hot