Preheat the oven to 180°C or 360°F. Line a baking tray with baking paper.
Tear off two 30cm pieces of baking paper and lay them flat on the prepared tray. Place the fish fillets in the middle of each paper.
Top the fish with the garlic, lemon slices, rosemary, olive oil, salt, pepper and chili flakes (if using). Scatter over the tomatoes.
Take each corner of the paper and fold into the centre to form a bag, then seal it.
Bake for 15-18 minutes, or until the fish is cooked through. Break open the bag just before serving so you can enjoy the fragances it releases. Spoon all the delicious juices over the fish and enjoy!