Preheat the oven to 200 degrees C. Cut out four 38cm squares of baking paper and fold each in half diagonally, forming triangles. Open the squares and coat with cooking spray.
Divide the spinach and onion among the paper squares. Cut the fish fillets in half crossways. Top each portion of spinach with 2 fillet halves, overlapping them slightly. Sprinkle each fillet with pepper.
Top the fillets evenly with tomatoes, carrots and spring onions. Drizzle each packet with 2 tablespoons wine or stock and 1/2 teaspoon of oil, then add 2 lemon slices and 1 thyme sprig.
Fold the baking paper over the filling to form a triangle. Fold the two open sides of each triangle over to seal. Fold over the points at the ends of the creased side, making a five-sided packet. Lightly coat the packets with cooking spray and place them on a baking tray.
Bake for about 15 minutes or until the packets puff. Transfer to plates and cut open the tops of the packets (be careful of the steam). Serve immediately.