Bring the juice to a simmer in a small saucepan over medium-high heat and reduce by half. Add the cream and cook until reduced enough to coat the back of a spoon, about 8 minutes.
Heat the butter in a skillet over medium-high heat. Add the mushrooms, thyme, garlic, and sherry and cook, stirring occasionally, until tender and almost dry, about 6 minutes
Combine the mushrooms with reduced gravy and season with salt and pepper. Reheat, if necessary, and stir in the vinegar. Transfer to a gravy boat and keep warm until serving.
George Stella's Livin' Low Carb: Family Recipes Stella-Style, Simon and Schuster, 2005