Preheat the oven to 200ºC, fan 180ºC , gas 6. Spread the mustard over the lamb. Mix together the parmesan and parsley and sprinkle over the top.
Put on a baking tray and cook for 20 minutes for medium, or 25-30 minutes for well done. Rest for 5-10 minutes.
Meanwhile cook the veg. Place the potatoes in a pan of cold water. Bring to the boil and cook for 10-15 minutes, until tender.Drain and mash with the butter and milk.Season with freshly ground black pepper. For the peas, cook in a pan of boiling water for 3-4 minutes until tender.
Drain, reserving 1 or 2 tablespoons of the water, and return to the pan. Add the mint, crème fraîche, lemon juice and reserved water, and crush with a potato masher.
Carve the lamb into cutlets and serve with the vegetables.