I have made and served this for so many years, I don't even remember where I happened to come by the recipe. I guarantee you there won't be any leftovers, so if you want some for yourself...might want to make a double batch. I will keep in the fridge for a week or maybe longer; we've never had it last longer.
In a large skillet melt the butter with the olive oil; when hot, add the onions. Stirring frequently, sauté onions on medium-high heat until they begin to brown and caramelize. Sauté in two batches if necessary. When ready, set aside.
In a nonreactive pot add the remaining ingredients and bring to a boil, then reduce heat to medium before adding the onions. Continue to cook until mixture thickens.
To serve, pour over a block of cream cheese and spread on crostini.