In a large bowl, combine the cabbage, apple, carrot and green onion. Set aside.
In a small bowl, whisk together the dijon mustard, vinegar, honey and olive oil. Season with salt and pepper. Pour the dressing over the slaw and toss to combine. Chill for 30 minutes or overnight.
Arrange the bread slices on a large cutting board and divide the mustard evenly between the bread slices spreading to coat the top of each slice.
Layer the cheese and pastrami evenly over four slices of bread.
Top each sandwich half with the slaw and then finish the sandwich by placing the remaining bread, mustard side down, on top of the slaw.