Gather your ingredients.(we used mixed green leaves in this image)
Squeeze as much liquid out of the grated sweet potato as possible using a clean cloth. Then place in a mixing bowl with the spring onion, flour, parsley, half the harissa paste and season. Mix together, adding a little more flour if needed. Shape into potato rostis.
Heat the olive oil in a large frying pan and fry the rostis for 4-5 minutes on each side, or until golden brown and crispy.
Meanwhile, poach the eggs. Bring the water to a rolling boil, swirl the water with a spoon and then crack the eggs into the water, poaching for 2-3 minutes (or until the whites are cooked through). Remove with a slotted spoon and drain on a kitchen towel or paper.
Spread the yoghurt (if using) across the plates and marble through some extra harissa. Top with the rostis, a poached egg and serve with a side of watercress or mixed green leaves.