To make the potato cakes, squeeze the excess moisture from the sweet potato, then combine with flour, parsley, egg white, harissa and some salt in a bowl until the mixture sticks together. Shape into 8 potato cakes. Heat a little oil in a frying pan. In two batches, fry the potato cakes for 2-3 mins each side or until golden brown and crisp. Keep warm while you repeat with the remaining cakes.
Poach the eggs in a saucepan of simmering water for 2-3 mins – the yolks should be runny. Remove with a slotted spoon and drain carefully on kitchen paper.
Spread some harissa in the middle of four serving plates and top with a dollop of yogurt. Add one potato cake to each plate then sandwich another potato cake on top with more yogurt. Top each stack with the remaining yogurt and an egg. Season and drizzle with more harissa and scatter over the micro herbs.