Heat a large ovenproof skillet over medium-high heat. Add 1 teaspoon oil to pan; swirl to coat. Pat onions dry with a paper towel. Add onions to pan; cook 2 minutes or until lightly browned, stirring once. Add apple to pan; place in oven. Bake at 400° for 10 minutes or until onions and apple are tender. Stir in 2 teaspoons butter, thyme, 1/4 teaspoon salt, and 1/4 teaspoon pepper.
Heat a large skillet over medium-high heat. Sprinkle pork with remaining 1/4 teaspoon salt and 1/4 teaspoon pepper. Add remaining 1 1/2 teaspoons oil to pan; swirl to coat. Add pork to pan; cook 3 minutes on each side or until desired degree of doneness. Remove pork from pan; keep warm. Combine broth and flour in a small bowl, stirring with a whisk. Add broth mixture to pan; bring to a boil, scraping pan to loosen browned bits. Cook 1 minute or until reduced to 1/4 cup. Stir in vinegar and remaining 1 teaspoon butter. Serve sauce with pork and apple mixture.