Add fennel and arugula to a large mixing bowl.
Add EVOO, grapefruit juice, balsamic glaze and a tiny pinch of salt and pepper to a small mixing bowl. Whisk vigorously to combine. Taste for seasoning and adjust salt and pepper as required. Note: this vinaigrette lacks an emulsifier so it is normal for the dressing to separate if set aside.
Pour vinaigrette overtop of salad and gently toss everything to coat. Transfer salad to serving platter and garnish with reserved fennel fronds, grapefruit segments, apple and pistachios. Serve on its own or alongside a lean protein of choice.