Rinse the fish (inside and out) in cold water, pat dry, salt. Mix the bread, onions, tomatoes, lemon zest and oil and season. Stuff the fish with this mixture.
Spread the potatoes and fennel on an oven tray lined with baking paper. Stir together the oil and the saffron and drizzle on the vegetables. Lay the fish on top.
for approx. 30 mins. in the centre of an oven preheated to 220°C. Remove and sprinkle with Fleur de Sel.