Peel the grapefruit, removing all of the white pith. Cut sides from the flesh of each grapefruit to form a square piece (reserve extra grapefruit trimmings for another use). Slice each square crosswise to form four slices. Set slices aside.
For dressing, in a small bowl whisk together apricot nectar, hazelnut or walnut oil, mint, salt, and pepper until combined.
Line a serving plate with spinach. Stack three slices of grapefruit, alternating colors, on top of spinach leaves. Drizzle with dressing. Sprinkle with walnuts and, if desired, pomegranate seeds. Makes 8 servings.