1.Preheat oven to 180°C.
2.Place lamb on a trivet in a baking dish. Brush with oil, sprinkle rosemary over and season to taste. Bake about 1 hour (allow 20-25 minutes per 500 grams).
3.Toss potatoes in extra oil and add to dish 45 minutes before end of cooking time. Remove lamb from oven, cover loosely with foil and rest for 20 minutes before carving.
4.Meanwhile, make mint jelly: In a medium saucepan, combine sugar, mint leaves, vinegar and water. Bring to boil on high, stirring until sugar dissolves. Reduce heat. Simmer 10 minutes. Stir dissolved gelatine through. Strain, discarding mint leaves.
5.Pour hot mixture into sterilised jars. Allow to cool until thickening, then stir extra fresh chopped mint through. Seal well. Chill until required.
6.Serve lamb with mint jelly and roasted potatoes.
The mint jelly recipe will make about 2 1/2 cups. Instead of using gelatine, you can substitute Jamsetta – available from supermarkets. It is powdered pectin and will allow the sauce to form a light gel.Note