MARINATED Chicken: Mix mustard and preserves in shallow non-reactive pan. Add chicken; turn to evenly coat both sides of each breast. Refrigerate 1 hour to marinate.
REMOVE chicken from marinade; discard marinade. Cook chicken in skillet sprayed with cooking spray on medium heat 4 to 5 min. on each side or until done (165ºF). Cut each breast into 1/2-inch-thick slices. Refrigerate until ready to use.
APRICOT Dressing: Mix ingredients until well blended. Refrigerate until ready to use.
FOR each serving: Top each of 2 toast slices with 1 Tbsp. Apricot Dressing, 1 ham slice, 2 oz. chicken, 2-1/2 Tbsp. onions and 1 cheese slice. Bake in 375ºF standard oven 10 min. or until cheese is melted. Arrange 2 lettuce leaves, 2 tomato slices and 4 apricots on serving plate; drizzle with 1/4 cup of the remaining Apricot Dressing. Serve with tartines on the side.