First thing in the morning, or last thing at night, crumble the fresh yeast (or pour the dried instant yeast) into the yogurt and stir well, by hand or with the paddle attachment of a mixer / beaters of an electric handwhisk. Add both the flours and the salt and stir together, then knead by hand or with a dough hook for 8 to 10 minutes until smooth. Place the dough (which is quite sticky) into a large bowl and cover with cling film (or a plastic shower cap!). Leave at normal room temperature for 8 to 12 hours.
In the evening, or the following morning, scrape out the dough onto a well floured surface, dust some more flour on top, turn over a couple of times and shape into a rough square, then roll out into a rectangle approx. 40 cm X 20 cm. Fold each short side in to meet in the middle, to form a 20 cm square. Turn over, then cut into 9 pieces and place them on 1 or 2 baking tray(s) (I use 2 as the rolls do expand) that are either lightly oiled, or lined with baking paper. With your fingers or a brush, dampen the tops with water, then scatter over the sunflower seeds. Cover loosely with lightly oiled cling film or a clean tea towel*** and leave in a warm place for 60 minutes until they have puffed up.
Preheat the oven to 250 C. If you don't have a water spray, place an empty roasting tin in the bottom of the oven. Put the rolls in the hot oven and either spray into the oven a few times with water, or pour a mug of boiling water into the roasting tin to create steam before closing the oven door.
Bake for 10 minutes at this temperature, then turn it down to 220 C and continue baking for a further 8 to 10 minutes until golden brown and crusty, and the base sounds hollow when tapped.
Deliciously crusty on the outside and soft in the middle, serve warm with butter, or leave to cool on a wire rack.