Heat oil and butter in a heavy skillet. Add onion slices and saute about 3 minutes.

Sprinkle in sugar, salt and pepper. Reduce heat to medium low and cook stirring often until onions are golden. This can take 20 minutes or more.

Stir in the vinegar, stock and whipping cream. Increase the heat and monitor until liquid thickens and is absorbed. This can be 5-10 minutes depending on the surface size of your skillet.

Stir in sage if using. Test and adjust seasoning as required.

Use as an appetizer topping or condiment with roasts and cheeses.