Add 2 teaspoons minced fresh thyme, if using, at end of cooking. Remove from the heat and transfer to a blender** (see Chef's Tip). Cover the blender tightly. Place a towel over the top of the blender and hold down. Pureeing hot liquids can be dangerous as they can burst from the blender and scald you. Puree the onions until smooth, adding a little water if the sauce is too thick.
Whatever the finished volume is of onions, add 25 percent Champagne vinegar and puree until smooth. Caramelized Onion sauce can be stored in refrigerator up to a month.
*Chef's Tip: you can replace vinegar with lemon juice.
**When blending hot liquids: Remove liquid from the heat and allow to cool for at least 5 minutes. Transfer liquid to a blender or food processor and fill it no more than halfway. If using a blender, release one corner of the lid. This prevents the vacuum effect that creates heat explosions. Place a towel over the top of the machine, pulse a few times then process on high speed until smooth.