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.