Preheat the oven to 180°C on Fan.
Put the peeled potatoes in a pot and pour plenty of cold water. Add salt and boil for about
10 minutes. Drain and set aside.
In a small bowl, mix together the butter, mustard, olive oil, thyme, onion, garlic, salt, and
pepper. Stir well.
Place the leg of lamb in a pan and dry it with paper towels. Brush the lamb all over with the
butter mixture.
Add the potatoes and the whole head of garlic to pan. Drizzle with olive oil and season with
salt and pepper. Add thyme and oregano.
Cover the pan with non-stick baking paper and seal with aluminium foil.
Place the pan in the oven and bake for 2 hours.
Uncover, turn the potatoes, and bake for another half an hour or until the meat is browned.