Preheat the oven to 180°C.
Process breadcrumbs, three tablespoons olive oil, mustard, butter, and herbs in a food processor until combined.
Season the lamb and sear in a hot ovenproof griddle pan with the remaining olive oil.
Press the herb crust onto the lamb and roast in the oven for 8–10 minutes.
Fry onion and garlic in a pan with oil until they begin to colour.
Add chopped potatoes to the pan and cook for another 3–4 minutes.
Reduce the wine in the pan, then simmer with cream, milk, and chives until potatoes are tender.
Slice vertical cuts into the whole potatoes without cutting all the way through.
Brown the potatoes in an ovenproof pan with oil, then roast in the oven for 8–10 minutes.
Plate the dauphinoise potatoes with the lamb on top and Hasselback potatoes on the side.