Preheat oven to 400°F and position rack in the lower-middle slot.
Wrap each beet in foil and roast until tender, 70 to 80 minutes.
Let cool for at least 20 minutes. Rub off skins with paper towels and dice into ½-inch pieces.
In a medium bowl, whisk garlic, vinegar, mustard, and olive oil until smooth. Season with salt and pepper.
Toss arugula and parsley with 2 tablespoons dressing and divide among plates.
Toss diced beets with 1 tablespoon dressing.
Arrange beets, walnuts, apple, and blue cheese over the greens.
Drizzle with remaining dressing and serve.