Prepare the seafood and produce: peel and devein the Large shrimp, leaving tails on if you like; halve the Straw mushrooms and cut the Roma tomatoes into wedges.
Smash and trim the woody ends of the Lemongrass and bruise or slice it into 2–3 inch pieces so it releases its aroma; thinly slice the Galangal and tear the Kaffir lime leaves to release their oils.
Lightly crush or slice the Small shallots and thinly slice the Thai bird's eye chiles — adjust the quantity of chiles to control the heat.
Bring Water (about 6 cups for 4 servings) to a gentle boil in a pot. Add the prepared lemongrass, galangal, kaffir lime leaves, shallots and chiles, then reduce to a simmer and let the aromatics infuse for 8–10 minutes.
Add the mushrooms and tomato wedges to the infused broth and simmer 2–3 minutes more so they soften but keep some texture.
Stir in a spoonful or two of Nam prik pao for depth and smokiness, then season with Fish sauce and a little Palm sugar to balance salty, sweet and savory — taste and adjust to your preference.
Add the prepared shrimp and simmer just until they turn opaque and curl, about 1–3 minutes depending on size; do not overcook.
Remove the pot from the heat and finish with freshly squeezed Fresh lime juice to brighten the soup. Taste and adjust with extra fish sauce, lime juice or palm sugar as needed.
Ladle into bowls and garnish generously with chopped Fresh cilantro and a few leaves of Sawtooth coriander. Serve hot with steamed rice or on its own.
