Grind together garlic, chilli powders and turmeric powder to a coarse paste.
In an earthenware vessel or pan, heat oil and add the mustard seeds. Allow to splutter and then add the fenugreek seeds, curry leaves followed by onion and ginger.
Sauté till the onions are softened and begin to take on a light brown colour. Next add the chilli paste, kokum pieces and 1 cup water. Season with salt and bring to boil.
Add the fish pieces and cook on medium heat for 5 minutes. Then lower heat and cook till done.
Garnish with curry leaves. Keep for at least 30 minutes before serving.