Cut the bottom tip off the lemon grass stalks and discard the loose outer layer(s). Then cut the stalks an inch apart at a slanted angle all the way up to the greener end where the grass blades start, exposing the inner core.
Smnash each piece with the side of a cleaver, or the end of a heavy handle to bruise, releasing the aromatic oils.
Place the cut lemon grass along with the sliced ginger, kaffir lime leaves, and about 2 quarts of waqter in a soup pot. Bring to a boil, reduce heat and simmer with a lid on for about 20-30 minutes to draw out flavors. Strain.
Add the bruised whole Thai chillies (or substitute0 and a slicd onion. Simmer a couple of minutes.
Add roasted chilli paste, tamarind jui9ce, and fresh brown mushrooms.
Return to a boil and simmer a couple more minutes.
Stir in the tomato wedges (if using), green onions and fish.
After 15-30 seconds, turn off heat, add lime juce to the desired sourness and the cilantro. Serve immediately. Do not let the fish overcook.