Heat 1 tablespoon of the oil in a large skillet over medium-high heat. Season the pork with ½ teaspoon salt and ¼ teaspoon pepper.
Cook pork until browned and cooked through, 5 to 6 minutes per side. Transfer to a plate.
Heat the remaining tablespoon of oil in the skillet over medium heat. Add the onion and cook, stirring occasionally, for 3 minutes.
Add the vinegar and cook until the onion is soft, 2 to 3 minutes.
Add the escarole and ¼ teaspoon each salt and pepper. Cook, tossing, until wilted, 2 to 3 minutes. Serve with the pork.