Place the saffron strands in a small heatproof bowl with 2 tbsp boiling water. Leave to soak for 5 minutes.
Meanwhile, heat 1 tbsp oil in a large deep frying pan. Fry the onion and fennel over a medium-low heat for 5-6 minutes until softened, adding the chopped garlic after 3 minutes.
Pour in the soaked saffron and water, tomatoes and stock and bring to the boil. Reduce the heat and add the cod, salmon and prawns. Simmer gently for 3-4 minutes, or until the fish is just cooked and the prawns are pink. Season to taste.
Lightly toast the bread slices, then drizzle over the rest of the olive oil and gently rub the cut sides of the garlic halves over the toast. Serve the stew in shallow warmed bowls with the garlic toasts and garnish with the reserved fennel fronds.