In a small skillet over medium heat, toast walnuts, stirring frequently until crisp and fragrant, about 5 minutes.
In a large bowl, whisk together the mustard, honey, shallot, and vinegar until well combined. Whisk in the oil until thick; season with salt and pepper.
Add apple and arugula; toss to coat. Arrange endive leaves on four serving plates, top with apple mixture and scatter mint, cheese, and walnuts over the top.