Heat the olive oil and butter together in a large skillet.
Add in the onions and start frying. Fry for 5 minutes to get them started then add in the mushrooms.
Fry until the mushrooms have reduced in size and lost their moisture and the onions are softened completely.
Add in the garlic and saute until browned and fragrant.
Stir in the balsamic vinegar and wine and cook another 2 minutes, making sure to delgraze the bottom of the pan ( scrape off any browned tasty bits so that it goes into the sauce!)
Season to taste with salt and pepper and serve!
Notes
If you don't want to use wine you can use a bit more balsamic vinegar to taste