Shred the Brussels sprouts: go to How to Shred Brussels Sprouts. Or, use a food processor slicing blade or mandoline to slice each sprout from top to end.
Prep the vegetables: Chop the escarole or other leafy green. Dice the apple.
Make the dressing: In a medium bowl, whisk together the Dijon mustard, white wine vinegar, maple syrup, and salt. Then whisk in the olive oil 1 tablespoon at a time until a creamy dressing forms.
Assemble the salad: Mix together all salad ingredients. Mix with 1/2 cup of the dressing (add more to taste, if desired). Top with feta crumbles, if using, and serve.