Preheat oven to 180°C/350°F. Pat the fish fillets dry with paper towels and season generously with salt and pepper. Set aside.
In a medium bowl, combine the tomatoes, olive oil, capers, lemon zest, parsley, oregano, vinegar, fish sauce and chili flakes along with a few generous grinds of pepper.
Set the fish on a large piece of aluminium foil. Spoon the tomato mixture on top of the fish. Crimp the foil tightly around the fish, then transfer to a rimmed baking tray.
Move to the oven and cook until the interior of the fish registers at 55°C/135°F, about 15-20 minutes. Allow to rest for 5 minutes before uncrimping and serving.