Soak the Dried long red chilies (prik chee fa) in hot Water for 20–30 minutes until softened, then drain (reserve a little soaking liquid).
Peel and halve the Small shallots; in a mortar and pestle or small blender, combine the softened chilies, the shallots, a spoonful of Shrimp paste (kapi) and a pinch of Coarse salt, and pound or blitz into a coarse, fragrant paste.
Bring about 3–4 cups of Water to a gentle simmer in a medium pot, then whisk in the Tamarind paste (concentrate) until dissolved to make a tart broth.
Season the broth with Fish sauce and a little Palm sugar, adjusting until you have a balanced sweet–sour–salty base (taste as you go).
Add the peeled and sliced Green papaya to the simmering broth and cook until just tender but still slightly crisp, about 5–8 minutes depending on thickness.
Scoop a few tablespoons of the hot broth into the mortar or blender to loosen the chili–shallot paste, then stir that paste back into the pot; the broth will take on a bright orange-red color and aromatic depth.
Gently add the Firm white fish (e.g., snakehead, sea bass, or cod) cut into large bite-size pieces and simmer only until the fish is cooked through, about 3–5 minutes so it stays moist.
Taste and adjust seasoning with more Fish sauce, Palm sugar or Tamarind paste (concentrate) as needed for the perfect balance; serve the gaeng som hot with steamed rice.
