In a large skillet, melt butter over medium heat. Add onion slices. Cook 10 minutes, stirring occasionally. Add apples; cook 15 to 20 minutes more or until apple and onion slices are tender and beginning to brown, stirring occasionally. Season with 1/8 teaspoon salt. Add vinegar; cook and stir for 30 seconds. Remove apple mixture from skillet; cover to keep warm.
While apple mixture is cooking, trim fat from meat. Cut meat crosswise into 1/2-inch thick slices. Sprinkle pork lightly with salt and pepper.
After removing apple mixture from skillet, heat oil in the same skillet over medium-high heat. Add pork; reduce heat to medium and cook for 6 to 8 minutes or until no longer pink, turning once.
To serve, spoon apple mixture onto the centers of 4 dinner plates. Top each with one-fourth of the pork slices. Serve immediately.