Rösti Stacks with Prawns and Mint Aïoli

Make mint aïoli. Blend or process mint, garlic, egg and mustard until mixture is pureed. With motor running, add oil in a thin, steady stream until aïoli thickens. Shell and devein prawns, leaving tails intact. To make rösti, squeeze excess moisture from potato; combine in medium bowl with rosemary and garlic. Divide into eight portions. Heat half the oil in large frying pan; cook rösti, in batches, flattening slightly, until browned both sides. Drain. Heat remaining oil in pan; cook prawns until just changed in color. Serve rösti stacked alternately with prawns, mint aïoli and arugula.