Peel the sweet potato and cut into rough chunks, then put in a microwaveable bowl with a splash of water. Cover with cling film, then cook on high for 8 mins. Meanwhile, whisk the tahini with the lemon juice and enough water to make a thick dressing. Season and set aside.
Carefully remove the cling film from the sweet potato, then blitz in a food processor. Leave to cool for a couple of mins, then add the coriander and coriander seeds. Blitz to a herby mash, then add the salmon, lemon zest, flour and some seasoning. Pulse again – you want the salmon to be roughly mixed in, rather than obliterated to mush.
Shape into four fishcakes. Chill in the fridge if you have time. Heat the oil in a large non-stick frying pan over a medium heat. Add the fishcakes and fry for 5 mins on each side until crisp and golden. Serve with the lemon wedges, tahini dressing and salad.