Finely dice the onions. Add the butter and oil to a pan set over medium heat. Once the butter has melted, add the onions and reduce the heat to low. Saute for 15-20 minutes until the onions are soft and caramelized. Stir intermittently to prevent from burning. Add the thyme sprigs
Add the bay leaf and apple cider vinegar to deglaze the pan. Stir well and saute for a couple of minutes.
Add the red wine to further deglaze the pan. Turn the heat up to medium. Add the vegetable stock, black pepper, dijon mustard and Worcestershire sauce (optional) and bring to a gentle simmer for 3 minutes for the flavors to amalgamate.
Remove the pan from the heat. Transfer the gravy to a blender. Blitz on high until a smooth consistency.
Return the gravy to the pan and set it over medium-low heat. Taste for seasoning. If your stock does not contain salt, you may wish to add ¼ teaspoon or, to your taste. If the gravy is too thick, add more water 1 tablespoon at a time to loosen it to your preference. Whisk well before transferring to a gravy boat before serving.