Boil sweet potatoes until tender. Drain and mash. Allow to cool.
Add eggs to sweet potato and mix well. Stir in breadcrumbs, feta, onions, fresh coriander, lentil flour, cumin, ground coriander, salt and pepper.
Using damp hands, form mixture into patties. Refrigerate until required.
Heat oil in a large frying pan over low heat. Add patties and cook for 5 - 6 minutes each side until golden. Remove to a paper towel lined plate and keep warm whilst cooking remaining patties.
For the yoghurt, combine all the ingredients and mix until smooth. Serve patties with yoghurt sauce, coriander and cucumbers.