Work spice, ginger, garlic and ⅛ cup of water into a loose paste using a mortar and pestle.
Fry onion in heavy based pan in hot oil until golden brown. Add paste and fry until it is fragrant and bubbling. Add tomatoes and simmer until the liquid has dissolved, then fry for 4-5 minutes or until fragrant. Stir in coconut milk and ⅓ of cup water, and simmer for 5 minutes.
Add fish and vegetables and cook until fish is tender, about 5 minutes.