Season the halibut lightly with sea salt and cayenne pepper on both sides. Heat the grapeseed oil in a large nonstick pan over medium heat. Place the fish in the pan and cook for 2½ minutes on one side; turn the fish and cook for another 2 minutes. Remove the fish from the pan; transfer onto a plate lined with paper towels and allow to rest.
Heat a large nonstick pan over low heat. Place the baby spinach and lemon zest into the heated pan. Drizzle with the olive oil and lightly stir. Season with salt and pepper.
Prepare fresh pomegranate juice, if necessary.* Combine the pomegranate juice and red port wine in a saucepan; simmer until reduced by a third. Add the orange juice, lemon juice, lime juice and grapefruit juice. Continue reducing until ½ cup of liquid remains. Add the olive oil. Combine all ingredients from Vinaigrette Garnish into the vinaigrette.
Divide the Spinach Garnish equally onto 6 plates. Spoon the POM-Citrus Vinaigrette around the spinach. Place each halibut fillet on top of the spinach.
*For 3 cups of juice, cut 6–9 large POM Wonderful Pomegranates in half and juice them with a citrus reamer or juicer. Pour the mixture through a cheesecloth-lined strainer or sieve. Set the juice aside.
*To prepare fresh arils, score 1 large POM Wonderful Pomegranate and place in a bowl of water. Break open the pomegranate under water to free the arils (seed sacs). The arils will sink to the bottom of the bowl and the membrane will float to the top. Sieve and put the arils in a separate bowl. Reserve ½ cup of the arils from fruit and set aside. (Refrigerate or freeze remaining arils for another use.)