Season lamb, roast at 220 degrees Celsius for 15-20 minutes, reduce to 180 degrees and cook for a further 45 minutes. Add the potatoes, parsnips and onions to the baking dish; toss these in the melted lamb fat; cook for a further 45 minutes.
Remove the lamb and rest, covered, for 15 minutes before serving. Meanwhile increase the oven temperature to 220 degrees to brown the potatoes, parsnips and onions.
If desired, the lamb can be served with a thin gravy: Make a brown roux with 30g butter and 1 level tblsp flour, add 1 glass of white wine, the pan juices (from which the fat has been skimmed), season and simmer for a few minutes. Alternatively, reserve some of the port and redcurrant jelly, add the pan juices, water or stock, season and simmer for a few minutes.
Port and Redcurrant Jelly
100g redcurrant jelly
1 tablespoon fresh mint, finely chopped (optional)
1 leaf gelatine, soaked in water for 5-7 minutes and squeezed out
In a small saucepan, gently heat the redcurrant jelly and port until the jelly has dissolved. Stir in the mint. Remove from heat, add the gelatine. Fill small moulds and place in the freezer to set.