Cut pita in half and spread the insides with mustard, dividing evenly between the two halves. Stuff the pita with apple and cheese, dividing evenly. Arrange on a foil-lined toaster-oven pan. Toast, flipping once if necessary, until the cheese is starting to melt and the pitas are hot but not burning, 4 to 5 minutes. Remove from the toaster with a spatula. Carefully add greens to the pita pockets, dividing evenly.