Preheat oven to moderate, 180C. Cut 4 large squares of baking paper.
In a bowl, combine tomatoes and onion, adding a little olive oil, if you like. Season to taste.
Place equal amounts of tomato mixture in the middle of each piece of baking paper. Top with fish. Sprinkle with dill sprigs. Drizzle with juice and, if you like, a little olive oil. Season to taste.
Wrap up as a parcel, place on a baking tray seam side up and tuck ends underneath. Bake 10-15 minutes until fish is cooked when tested.
Remove from parcel and serve with salad or vegetables.
