Mix the flour with 1/2 tsp salt and make a well in the centre. Crumble in the yeast.
Add 250ml lukewarm water and knead to a smooth, supple dough. Cover and leave to rise in a warm place for 30 minutes.
Meanwhile, drain the tomatoes and cut into strips. Wash the rosemary, shake dry and chop the needles.
Thinly slice the fish fillet.
Drain the mozzarella and cut into slices.
Divide the pizza dough into 4 portions and roll out each one to a 23cm round.
Place 2 pizzas on each baking tray lined with baking paper.
Spread some tomatoes on the pizzas. Add the rosemary, salt and pepper. Cover the pizzas with mozzarella slices and bake in a preheated oven at 220°C (200°C fan, gas 7) for 15 minutes.
Add the fish and tomato strips to the pizzas and bake for another 5 minutes.
Season the pizzas with salt and pepper before serving.