Preheat the oven to 200C/400F/Gas 6.
For the dressing, place the mustard, white wine vinegar, groundnut oil, olive oil and parsley into a bowl and whisk well. Add the roughly chopped hazelnuts, stir together and season well with salt and freshly ground black pepper.
Meanwhile, for the lamb, heat a frying pan until hot, then add the oil and butter. Once the butter is foaming add the lamb and fry for 1-2 minutes, until browned. Remove from the pan and cool.
Mix the mustard and white wine together in a bowl, then brush the mixture over the lamb to evenly coat.
Spread the finely chopped hazelnuts out onto a baking tray and roll the mustard-coated lamb in the hazelnuts to coat.
Place the coated lamb onto a wire rack in a roasting tray and place into the oven to roast for 8-10 minutes, until just pink (cook for longer according to your preference). Remove from the oven and rest for five minutes.
For the sautéed potatoes, heat a frying pan until hot and add the butter and potatoes. Sauté the potatoes, turning occasionally, until golden brown.
Add the chopped shallots and season, to taste, with salt and freshly ground black pepper.
To serve, lay the potatoes in the centre of a plate. Slice the lamb and lay on top of the potatoes. Season the salad leaves with the hazelnut dressing and place alongside.