1.Preheat a char-grill to moderately high. Brush eggplant, capsicum and zucchini with half the oil; cook vegetables, in batches, 2-3 minutes each side, or until tender. Transfer to a heatproof plate; cover with toil.
2.Combine kumara, egg, flour and spread in a medium bowl; season.
3.Heat remaining oil in a large non-stick frying pan over moderate heat.
4.Divide kumara mixture into 8 portions (about 1/3 cup each). For each rosti, cook 1 portion of kumara mixture 2-3 minutes; turn and flatten with an egg lifter. Cook 2-3 minutes more, or until crisp and golden.
5.Whisk pesto and yoghurt in a small bowl. Layer rosti, vegetables and feta on serving plates; top with pesto yoghurt. Serve.