Heat the oil in a large saucepan and gently fry the onion and leek for 5 minutes. Add the garlic and fry for a further 1 minute.
Stir in the saffron, tomatoes, tomato paste, stock, bay leaves and thyme. Bring to the boil, then reduce the heat and simmer gently for 25 minutes.
Gently stir in the mixed fish and cook very gently for 5 minutes. Stir in the prawns and cook for a further 2–3 minutes until the prawns have turned pink and the fish flakes easily when pierced with a knife.
Check the seasoning and ladle the stew into shallow bowls. To garnish, spoon some aïoli on to baguette slices and rest them over the stew.