This is a nice summery dish and serves well warm or cold.

1 Tablespoon Butter

1 Tablespoon Olive Oil

2 x Medium Red Onions (thinly sliced preferably with a mandolin)

2 lb New Potatoes (par boiled)

1 x Tablespoon Soft Brown Sugar

1 Tablespoon Balsamic Vinegar

 Gas 4; 180C

Once, par boiled, add the potatoes to a roasting dish and spread the onions over the top.  
Sprinkle over the rest of the ingredients and pop into the oven for about an hour, stirring occasionally.