Heat the oil and 1 Tbsp of the butter in a skillet over medium high heat on the stove.

Add the mushrooms and onion to the skillet, and let sit for 2 minutes. Stir, and then saute another 6-7 minutes until tender.

In a bowl, whisk together the balsamic vinegar, water and brown sugar until combined.

Add the garlic to the skillet as well as the salt and pepper. Saute another minute.

Reduce the heat to low, pour the sauce over the mixture, and toss to combine.

Cook another minute or two before serving.