In a small saucepan, whisk together vinegar, mustard and honey. Set pan over medium-high heat and bring to a simmer. Simmer until liquid reduces to 1/4 cup, about 3 minutes.
Spread mixture on toasted or grilled challah bread slices Top with Brie and then chicken. Using a vegetable peeler, shave apple into thin slices and arrange over chicken. Top with second slice of bread.