COMBINE onions and dressing. Refrigerate 30 min. or until ready to use.
FOR each serving: Spread 1 Tbsp. Lemon-Mustard Cream Cheese Spread onto bottom half of split Focaccia Bread. Top each with about 4 mushroom slices, 1 Tbsp. peppers and about 1/2 Tbsp. onions; cover with top of bread. Pipe 1 tsp. Lemon-Mustard Cream Cheese Spread onto top of each sandwich; top with 2 tomato pieces and 1 rosemary sprig.