Grind the whole spices to a fine powder (see notes)
Cut green pumpkin and potatoes into semi-circles approximately 7mm wide.
Brush the fish steaks with a little flour, season with lemon juice and leave to marinate for 30 minutes, then wash the flour off (this will help get rid of the fishy smell).
Add onions and garlic to a large non-stick skillet or casserole, pour in water to just cover and boil over medium heat for 15-20 minutes until soft.
Add the chopped tomatoes and the spice mix and continue simmering for 10 minutes, adding water if necessary.
Smash or blitz the mixture to smooth using a hand blender.
Add the salt, green chili and potatoes and cook for 5 minutes.
Add green pumpkin and arrange the fish steaks on top.
Pour in a little more water if needed, cover and continue to cook until the fish and vegetables are done, for about 20 minutes (depending on the size of the pieces). Taste and adjust for salt.
Serve garnished with fresh coriander leaves and lots of lemon juice.