Boil sliced onion and cashews in water for 8 minutes; blend to a smooth paste.
Heat ghee in a pan, add ginger paste, garlic paste, and nut-onion paste; cook for 4 minutes.
Whisk yogurt with salt and cardamom powder; pour into the gravy on low flame.
Add fish pieces and water; cover and simmer gently for 8 minutes until fish is cooked.
Serve warm.