Bring the juice to a simmer in a small saucepan over medium-high heat and reduce by half.
Add half & half 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 serve warm.