In a large skillet, melt butter over medium heat. Add onion slices. Cook 5 to 6 minutes, stirring occasionally.
Add apples; cook 10 to 15 minutes more or until apple and onion slices are tender and beginning to brown, stirring occasionally. Season with ¼ teaspoon salt.
Add vinegar and thyme to the skillet; cook and stir for 30 seconds. Remove apple mixture from skillet; cover to keep warm.
Meanwhile, prepare pork. Cut meat crosswise into ½-inch thick slices. Sprinkle pork with ½ teaspoon salt and ¼ teaspoon 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 just barely pink in the centers, turning once.
Remove pork from pan and allow meat to rest 2 to 3 minutes before serving. Serve pork tenderloin topped with sautéed apples and onions.
Video
Notes
Look for a flavorful apple that holds its shape when cooked, like Honeycrisp, Granny Smith, Fuji, Braeburn or Sweet Tango, to name a few.