Meanwhile, in 10-inch skillet, melt butter over medium heat. Add onion and salt and cook, stirring frequently, 5 to 7 minutes or until edges of onion slices are browned and soft. Add mushrooms; continue to cook 5 to 7 minutes or until mushrooms release juices, liquid evaporates and mushrooms brown. Add vinegar and stir, scraping up bits from bottom of pan until almost completely evaporated. Add pork, and stir to combine. Add cream and reserved pasta water, and continue to cook 2 to 3 minutes or until thickened. Remove from heat.