Dice the onion, carrot and capsicum.
Cut fish into 3cm cubes. Season fish with salt and pepper.
Heat oil in large saucepan over medium heat. Add onion and cook, stirring, until softened, then add butter, carrot and capsicum and fry for 5 minutes until softened. Add garlic and cook for one minute until fragrant.
Add tinned chopped tomatoes, stock, lemon rind and chilli flakes.
Bring to the boil then reduce heat, cover and simmer for 15 minutes until soup reduces and thickens slightly.
Meantime, put baguette thins on baking paper, drizzle small amount of olive oil across them and place in a 200℃/392℉ oven for about 10 minutes. When crispy, remove from oven and rub garlic clove across each just once. (Too many rubs and garlic will be overpowering.)
Add fish to tomato and stock, and cook uncovered for three minutes before adding prawns. Cook everything, stirring gently, for a further two to three minutes or until seafood changes colour and is cooked.
Add herbs, and further seasoning.
Ladle into bowls and serve with crispy baguette if using.