Heat oil in 12-inch nonstick skillet over medium-high heat until shimmering. Add shallot and cook until softened and starting to brown, about 2 minutes. Add mushrooms and 1/8 teaspoon salt and season with pepper to taste. Cook until liquid has evaporated and mushrooms begin to brown, 6 to 8 minutes. Transfer mixture to bowl and stir in vinegar.

Makes 3/4 cup