FOR each serving: Place 1/2 oz. Brie on 1 bread slice; top with 1/4 cup each onions and mushrooms, and 2 oz. each ham and chicken. Spread 1 Tbsp. bistro sauce onto second bread slice; place over sandwich.
MELT butter in skillet on medium-low heat. Add sandwich; cook 1-1/2 min. on each side or until golden brown on both sides, occasionally using back of spatula to firmly press down on sandwich as it cooks.