Remove all skin from fish fillets, ensure that no bones remain and cut into serving-size pieces. Chop very finely the shallots, ginger, garlic and chiles, and pound these together with the turmeric powder, tamarind and soy sauce (or use a food processor). Heat the oil in a shallow pan and stir-fry the spice-paste for four to five minutes, then add the fish, cover with approximately one cup of cold water and bring to a boil. Lower heat, season to taste with salt and freshly ground black pepper and cook over a very low heat until the fish is done. Serve with fresh vegetables and rice.