Mix together the yogurt, harissa, coriander and lime rind and juice in a non-metallic bowl. Add the salmon and coat in the mixture. Cover and marinate in the refrigerator for at least 20 minutes.
Toss together the sweet potato chunks, olive oil, cumin seeds and garam masala in a bowl and season well. Put in a roasting tin and place in a preheated oven, 200°C (400°F), Gas Mark 6, for 35–40 minutes until golden.
Heat a lightly oiled frying pan or griddle until hot towards the end of the sweet potato roasting time. Add the salmon and cook for 3 minutes on each side until just cooked. Garnish with coriander and serve with the sweet potatoes, lime wedges and flat-leaf parsley.