Place shallots, garlic, ginger, fennel seeds, ground turmeric, ground coriander and red chili powder in a food processor. Add a little water and process to a smooth paste. Set aside. 

If using tamarind blocks, soak in water for about 10 minutes, squeeze out the pulp and discard the hard bits. Set aside.

Heat oil in a large frying pan or skillet over medium-high heat.

Add onions, green chilies and curry leaves. Sauté, stirring occasionally, for 2-3 minutes or until browned.

Add tomatoes. Sauté, stirring occasionally, for 2 minutes.

Add ground paste, fish, tamarind, coconut milk and water. Stir to combine. Season with salt. Simmer for 15-17 minutes or until fish is cooked through. 

For tempering, heat oil in a small pan over medium heat. Add mustard seeds. Add shallots, dried red chilies and curry leaves. Sauté, stirring, for 1 minute or until shallots are browned.

Add tempered spices to the curry. Stir to combine.