ONION PUREE

MAKES ABOUT 1 CUP

I DO NOT THINK OF THIS PUREE as a substitute for butter, but I often use it at the point in a classic recipe where butter is called for to add creaminess and mellow flavor.

3 tablespoons olive oil

2 onions, halved and very thinly sliced

1 tablespoon sherry vinegar

Salt and pepper

Heat the oil in a medium saucepan over medium heat. Add the onions and sweat, stirring often, until they are completely soft but not browned, 15 to 20 minutes. Add the vinegar and cook for about 5 minutes more. Transfer the mixture to a blender and process until satiny smooth, then add salt and pepper. The puree will keep, covered in the refrigerator, for up to 3 days.