Stir in ham, salt and pepper; cook uncovered 5 to 7 minutes, stirring occasionally, until browned. In small bowl, mix water and mustard; stir into potato mixture. Make 4 holes (about 2 inches each) in mixture. Crack an egg into each hole, and drizzle eggs with vinegar. Sprinkle cheese over potato mixture (not over eggs), and cover. Cook 3 to 5 minutes or until eggs are set. Sprinkle with chives.