Melt the butter in a large heavy-based saucepan over low heat. Stir in the onion and cook, covered, stirring occasionally, for 50 minutes or until soft and lightly coloured. Add the flour, sage and salt and pepper to taste and cook for another 4 minutes. Add the wine and cook for 2–3 minutes or until it has evaporated.
Add the stock and return to the boil, then reduce the heat to low an