For the röstis, heat a heavy-based frying pan.
Place the grated potato into a bowl and season well with salt and freshly ground black pepper. Divide into four portions.
Place the oil and butter into the heated frying pan, then add the potato patties. Cook for 3-4 minutes on each side, pressing them down firmly with a fish slice, until cooked through and golden-brown.
Meanwhile, for the prawns, heat another frying pan. Add the olive oil and tip in the prawns. Fry over a medium heat for about a minute, until the prawns have turned pink and are cooked though.
Squeeze over the lime juice and sprinkle over the dill. Season, to taste, with salt and freshly ground black pepper and stir well.
For the mustard soured cream, place the yoghurt in a small bowl with the cream, mustard, honey, lemon zest and olive oil. Season, to taste, with salt and freshly ground black pepper and stir well.
For the herb salad, place the herbs into a bowl and drizzle over the vinegar and oil. Season, to taste, with salt and freshly ground black pepper and stir to coat the leaves.
To serve, place one of the potato röstis onto a warmed plate and spoon some of the prawns on top. Drizzle over a little of the mustard soured cream. Repeat these layers until all of the ingredients have been used up. Garnish with the dressed herb salad and serve at once.