Trim any fat from pork. Cut pork crosswise into 1/2-inch-thick slices. Spray a large nonstick skillet with nonstick coating. Preheat skillet over medium-high heat. Cook half of the pork in the hot skillet for 3-1/2 to 4 minutes or until pork is slightly pink in the center and the juices run clear, turning once. Remove pork from skillet; keep warm. Repeat with remaining pork.
Carefully add oil to skillet; add the onions. Cook, covered, over medium-low heat for 13 to 15 minutes or until onions are tender. Uncover; stir in the brown sugar. Cook and stir over medium-high heat for 4 to 5 minutes or until onions are golden.
Meanwhile, in a small bowl stir together water, vinegar, cornstarch, salt, and pepper; carefully stir into onion mixture in skillet. Cook and stir until thickened and bubbly. Cook and stir for 2 minutes more. Return pork to skillet; heat through.
To serve, transfer the pork and onion mixture to a serving platter. Sprinkle with parsley. Makes 4 servings.