For the hogget, preheat the oven to 200C/390F/Gas 6.
Season the hogget with salt and freshly ground black pepper.
Heat a large frying pan until hot and add the olive oil.
Place the hogget in the pan and fry on both sides until browned.
Place the chopped onion into a roasting tray and arrange the hogget on top.
Transfer the tray to the oven and cook for 1 hour 15 minutes (which is 30 minutes per kilo (2lb 2oz), plus an extra 30 minutes).
Remove the hogget from the oven and rest, covered in foil, in a warm place for at least 15 minutes before carving.
For the dauphinoise potatoes, preheat the oven to 175C/350F/Gas 3.
Place the potatoes into a bowl of cold water and swirl around to remove any excess starch. Drain well and pat dry.
Butter an ovenproof dish with about a teaspoon of the butter and place the potatoes in the dish in layers, overlapping a little, until the dish is just full - season with salt, pepper and nutmeg between each layer.
Place the cream and milk into a bowl and whisk to combine.
Season with salt, black pepper and nutmeg and pour the cream and milk over the potatoes.
Dot with the remaining butter and then cover tightly with aluminium foil.
Place in the oven and cook for one hour until the potatoes are just tender.
After an hour, carefully remove the foil and return to the oven for a further 30 minutes until golden-brown on top.
For the parsnips, preheat the oven to 200C/390F/Gas 6.
Mix the parsnips with the olive oil and season with salt and freshly ground black pepper. Place into a roasting tray.
Place in the oven and roast for 30-40 minutes until tender and golden-brown.
For the mint sauce, chop the mint leaves. Sprinkle the salt over the mint and chop even more finely.
Place the mint and salt into a jug, add the sugar and pour over the boiling water. Stir, add the vinegar and leave to cool.
Serve the hogget in slices along with the dauphinoise potatoes, parsnips and a drizzle of mint sauce.