Heat 1 Tbsp. oil in large nonstick skillet over medium-high heat. Add onion, bell pepper, and garlic; cook, stirring occasionally, until tender, 3 to 4 minutes. Add the mushrooms; cook, stirring occasionally, until browned, 3 to 5 minutes. Add vinegar; cook, stirring and scraping up any browned bits, until the liquid has evaporated, 1 to 2 minutes. Let cool for 10 minutes.