Instructions: Preheat the oven to 200 C/400 F/gas mark 6.

To make the starter: Sprinkle the yeast into the water. Leave for 5 minutes, stir to dissolve. Add the flour and mix to form a thick batter.

Cover with a tea towl and leave to ferment at room temperature for at least 12 hours and up to 36 hours, unitl it forms a loose, bubbling batter.

To make the dough: Sprinkle the yeast into 100ml (3 1/2fl oz) of the water.

Leave for 5 minutes, stir to dissolve. Mix the flour and salt in a large bowl. Make a well in the centre of the flour. Pour in the yeasted water, the olive oil and the starter.

Mix in the flour. Stir in the remaining water to form a soft, sticky dough.

Add extra water one tbsp at a time if necessary.

Turn out on to a lightly floured work surface. Knead the dough until smooth, silky and elastic, about 10 minutes. Put the dough in a clean, lightly oiled bowl and cover with a tea towel. Leave to rise until doubled in size, 1 1/2-2 hours. Knock back and chafe for 5 mintues then leave to rest, about 10 minutes.

Roll out the dough on a lightly floured work surface to form a round, 24cm

(9 1/2 inches) in diameter. Place the dough on a lightly oiled baking sheet and cover with a tea towel. Prove until doubled in size, about 30 minutes.

To make the topping: Use your fingertips to gently press into the dough to make dimples about 1 cm deep. Scatter an even layer of onion wedges and Gruyere on top of the dough, then sprinkle with thyme, oil and salt.

Bake in the preheated oven for 30 minutes until puffed and the topping is crisp. Leave to cool slightly on a wire rack. Cut into wedges and serve while still warm. It is also good at rooom temperature.

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