To prepare and roast lamb leg: Mix together all the ingredients for the seasoning butter and use to coat the lamb leg. Place seasoned lamb leg in a roasting pan and allow to marinate for 1 hour in the refrigerator.
Ideally, roast the lamb leg using a rotisserie. Roast in preheated oven at 160ºC for about 2 hours, or until the juice runs clear when pierced with a skewer (for every kilogramme of meat, allow 30 minutes cooking time). Baste with the pan juices every 10 minutes. Remove from heat and rest for 30 minutes in a warm place.
To prepare garlic sauce: Peel the garlic and simmer for 20 minutes in salted water. Drain and process in the blender with white wine (or water), sugar, salt and pepper. Transfer mixture to a saucepan. Add egg yolks and place the pan over a low fire. Whisk briskly until the mixture doubles in volume and becomes creamy.
To serve, carve the lamb and spoon a little garlic sauce over each portion, or serve sauce on the side.