Cut tenderloin crosswise into 8 pieces. Place each piece between 2 sheets of heavy-duty plastic wrap; pound each piece to 1/2-inch thickness using a meat mallet or small, heavy skillet. Combine salt, thyme, pepper, and allspice; rub over both sides of pork.

Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Add pork to pan, and cook for 2 minutes on each side or until done. Remove pork from pan; keep warm.

Reduce heat to medium. Add shallots and butter to pan; cook 2 minutes, stirring occasionally. Add brown sugar and vinegar to pan; cook 30 seconds or until sugar melts, stirring constantly. Spoon sauce over pork.