Dissolve the yeast in the water, then add the yogurt and milk. In a food mixer with dough hook attachment, place the flour, fermenté and mashed potato. Pour in the yeast mixture and mix to a solid dough. Mix on low for 6 minutes. Add the salt, then mix on medium speed for a further 3 minutes.
Place the dough in a bowl, cover with a damp towel and leave until the dough has risen by half