I made my San Francisco sourdough starter to the exact recipe from Baking with Passion, and so with Dan Lepards permission I reproduce it here.
“Our San Francisco sourdough starter, which is a poolish, uses yoghurt, as is typical in American artisanal baking. The polish will raise dough as soon as the fermentation has begun, but we find that it takes a couple of weeks for a real flavour to develop. Bakers will give you pieces of wild yeast starter that have a developed flavour, and these can be beaten into the batter just before you add the fruit.
Vigorous and prolonged beating is important when making the starter, to incorporate the maximum amount of air. This will help to ensure that fermentation occurs quickly and aggressively. Later on, this activity is deliberately slowed as the emphasis shifts away from the generation of carbon dioxide needed to raise the loaf and towards the development of flavour . . . →San Francisco sourdough starter