Mix the flour and 1/4 tsp salt in a bowl. Make a well in the centre and crumble in the yeast.
Add 250ml lukewarm water and mix with a dough hook on a hand mixer to form a snooth dough. Rest in a warm place for about 30 minutes.
Meanwhile peel and slice the onion. Wash the tomatoes, remove the stalks and slice.
Rinse the anchovy fillets and capers under cold water and drain.
Divide the pizza dough into 8 portions and roll out each one on a floured surface to a 12cm round.
Place 4 dough circles on a sheet of baking paper and place on a baking tray. Repeat for the remaining 4 circles.
Spread some tomato puree on the pizzas. Cover with tomato slices, onion rings, anchovies and capers and season with salt and pepper.
Sprinkle over the cheese and place on the lowerest shelf in a preheated oven at 225°C (200°C fan, gas 7).
Meanwhile, wash the parsley, shake dry and roughly chop the leaves.
Zest the lemon. Peel and finely chop the garlic, then mix it with parsley and lemon zest.
Remove the pizzas from the oven and serve with the herb mixture (gremolata).