Kasundi mustard paste is one of those ingredients that rewards a light hand. Too much and the curry turns bitter; the right amount and it gives the whole pot a sharp, tangy backbone that plain mustard oil never quite reaches.
Start by lightly frying the fish pieces in mustard oil until just golden, then set them aside — this keeps them from breaking apart later. In the same oil, cook onion, ginger, garlic and green chilli until soft.
Whisk two tablespoons of kasundi into a little water before adding it to the pan — this stops it from splitting under high heat. Simmer, slide the fish back in, and let everything come together for five minutes.
Serve over plain rice with a wedge of lemon on the side. If the curry tastes flat, it usually needs more kasundi rather than more salt.
