Mix the lemon zest, dill, salt and pepper, and use to season the fish.
To make the rolls
Blanch the cabbage leaves in salted water for approx. 30 secs. per batch, remove with a slotted spoon, dip briefly in ice-cold water, drain, pat dry, remove the stalk.
Arrange the fish on top of the leaves, fold the sides of the leaves inwards, roll up.
Sweet potato mash
Cook the sweet potatoes in salted water for approx. 20 mins. until soft. Drain the sweet potatoes and return to the pan, mix in the butter. Mash the sweet potatoes with a fork, season.
Pangasius and cabbage rolls
Bring the stock and cream to the boil, add the rolls, cover and cook over a medium heat for approx. 15 mins., remove and keep warm.
Stir the cornflour into the water, add to the stock, simmer for approx. 3 mins., return the rolls to the pan and heat gently. Garnish with a little dill.