Combine the balsamic vinegar, brown sugar, and water in a small prep bowl and set near the stove.
In a large skillet, heat 1 tablespoon of the butter with the olive oil over medium-high heat.3
When the butter is melted, add the mushrooms and the salt and stir right away with a wooden spoon until the mushrooms have absorbed all the fat from the butter and oil.
Let the mushrooms cook undisturbed for 2 minutes and then stir once. Keep the heat at medium-high and continue to cook, stirring occasionally, until the mushrooms are shrunken, glistening, and some sides have developed a deep orange-brown color, 6 to 7 minutes more.
Reduce heat to low, add the garlic and the remaining butter and cook, stirring, until the butter is melted and the garlic is fragrant, 15 to 20 seconds.4
Carefully add the balsamic mixture and cook until the liquid reduces to a glaze consistency that coats the mushrooms, about 15 to 20 seconds. Stirring frequently. Season with black pepper.
Immediately transfer the mushrooms to the serving dish, scraping the pan with a rubber spatula to get all of the garlicky sauce. Let sit for a few minutes and then serve warm.