MELT 1 tsp. butter in large skillet on medium-high heat. Add mushrooms; cook and stir 5 to 6 min. or until tender. Remove from skillet.
FILL bread slices with mushrooms and remaining ingredients to make 2 sandwiches. Spread outsides of sandwiches with remaining butter.
COOK in skillet on medium heat 3 min. on each side or until golden brown on both sides.