Prepare the vinaigrette: In a small jar or bowl, combine the olive oil, apple cider vinegar, Dijon mustard, honey, garlic, salt, and pepper. Whisk or shake until smooth and creamy.
Layer the salad: Spread the arugula on a large platter or shallow bowl. Scatter the cooled farro evenly over the greens.
Add toppings: Arrange the sliced apple across the salad, then sprinkle on the walnuts and Parmesan.
Dress the salad: Drizzle about half the vinaigrette over the top. Toss lightly to combine, then add more dressing to taste as needed.
Serve immediately, garnished with fresh parsley or a squeeze of lemon, if desired.
