In a saucepan, soften the onions in the butter over high heat for about 10 minutes. Add the sugar and vinegar. Cook for 10 minutes. Add the wine and cook over very low heat for about 1 hour 15 minutes, stirring occasionally. Season with salt and pepper. The liquid should have completely evaporated.
Serve with pâté or terrine, as our guinea fowl, apricot and pistachio terrine.
The onion jam is also perfect to garnish duck breast crostini.