Add the oil, garlic and mushrooms to a frying pan and saute for a few minutes.
Add half the balsamic vinegar to the pan and turn up the heat so it bubbles and reduces. When it's reduced quite a bit, add the rest of the vinegar. Keep it bubbling but don't let it reduce to nothing.
Take it off the heat and add the cream, salt and pepper.
Reduce the heat and cook until the cream is hot - you may want to reduce it a bit more.