Preheat the oven to 375 degrees. In a large mixing bowl, toss together the carrots, red onion, mandarin juice, olive oil and vinegar. Season with a pinch of sea salt and pepper. Cut the fish fillet in half. Place each piece on one side of a square of parchment paper and top with the carrot mixture and any remaining orange juice. Season with a pinch of sea salt and pepper.
Seal the parchment packs by folding over the square. Fold the parchment tightly along the outside edge, curving around the fish and sealing it inside. Place in the oven on a baking sheet for 8-10 minutes until the fish is cooked through and completely opaque.