04Put the fennel, its cooking liquid and the reserved fennel fronds in the bag with the fish. Season the tomatoes with salt and pepper and add them to the bag. Push down on the bag to squeeze out excess air and seal with string.05Carefully place the bag in an oval baking dish or shallow pan (it's easiest to bake the fish in something you can take straight to the table) and bake for about 15 minutes per inch of thickness (about 30 minutes for a 2-inch-thick fish).06To serve, cut the bag open with a knife or scissors and serve pieces of fish surrounded by fennel, chopped tomatoes and a spoonful of the cooking juices.