Boil sliced onion and cashews in water for 8 minutes, cool, and blend to a smooth paste.
Heat ghee in a skillet, sauté ginger paste, garlic paste, and nut-onion paste for 4 minutes.
Whisk yogurt with salt and cardamom powder; pour into skillet on low flame.
Add fish pieces and water; cover and simmer gently for 8 minutes until fish is cooked through.
Serve warm with basmati rice or naan.