Spread mustard on pocket breads. Cut each pocket bread into 6 triangles; place on ungreased cookie sheet. Cut each cheese slice diagonally into 4 triangles; place 1 cheese triangle on each bread triangle. Arrange mushroom slices over cheese. Sprinkle with Italian seasoning.
Broil 4 to 6 inches from heat for 2 to 4 minutes or until cheese is melted. Serve warm.