Peel and dice the sweet potatoes. Slice the leek or onion. Deseed then roughly chop the red pepper.
Heat the oil in a large, lidded pan - add the sweet potatoes and leek, put the lid on and gently sauté for about 8 minutes, until they are tender.
Add the pepper and tomatoes and stir for a couple of minutes.
Add the spices, then stir and the lemon juice together with a drop of water if the vegetables are too dry. Cook for another 8-10 minutes (under cover), until the tomatoes burst and start releasing their juices.
Cut the haddock into bite-sized chunks and lay these on top of the vegetables ( which should be soft by now). Season with salt and pepper generously, put the lid on and cook for about 5 minutes until the fish changes colour. Turn off the heat, and let the fish steam (under the lid) for a couple of minutes (to make sure it's cooked through).
Serve with coriander scattered all over and some extra lemon/lime juiced squeezed on top.