Preheat the oven to 200 °C. Place the seafood, olive oil, lemon juice and thyme in a roasting bag and season well with salt and pepper.
Tie the bag tightly and gently massage the pieces of seafood to evenly distribute the seasoning.
Place the bag on a roasting tray. It is vital that the fish is in a single, flat layer inside the bag, evenly spaced to prevent sogginess. Bake for 15 minutes.
Meanwhile, make the soup base: heat the olive oil in a large saucepan and gently sauté the onion and garlic until soft and glassy. Add the tomatoes, herbs, red pepper flakes (if using) and liquids and simmer gently until thickened and aromatic.
Taste for seasoning and add sugar if the soup is too tart.
Remove the roasting tray from the oven and, with scissors, make an incision in the bag to let the steam escape.
Swiftly cut open the bag and disperse the contents gently into the soup base.
Gently stir to mix and simmer for 5 minutes to allow the flavours to marry.
Serve immediately, accompanied with garlic crostini.