Combine first 4 ingredients in a small bowl; set aside.
Sprinkle both sides of pork with salt and pepper. Place a large nonstick skillet over medium-high heat until hot. Coat skillet with cooking spray; add pork, and cook 4 minutes on each side. Remove pork from pan, and keep warm.
Add cider mixture to pan, scraping pan to loosen browned bits. Bring to a boil; add onion, and cook 2 minutes. Reduce heat to medium, and add plums; cook 2 minutes. Add pork, and simmer 6 minutes. Turn pork; cook 6 minutes or until pork is no longer pink and onion mixture is reduced to 1 cup. Serve onion mixture over pork.