To make the fruit soak, put the currants and raisins in a bowl and pour over the water. Cover with plastic wrap and soak the fruit overnight. Drain the fruit, discarding the water, and set aside until needed.
Follow the instructions for mixing a sourdough loaf until you can create a window. Measure 125 g (4½ oz) of the fruit soak and combine with the hazelnuts and rye starter. Lightly mix through the dough until just combined. You can do this by hand by lightly folding the ingredients through the dough or simply add it to the bowl of an electric mixer and mix for 2–3 minutes on slow speed — you will need to give it a helping hand by stopping the mixer a few times to push the dough around the bowl. Lightly grease a container with oil spray and sit the dough inside. Cover with plastic wrap and set aside at ambient room temperature (approximately 20°C/68°F) for 1 hour to bulk prove.
To knock back the dough, turn it out onto a lightly floured work surface and press out into a rectangle, about 2.5 cm (1 inch) thick. Use your hands to fold one-third back onto itself, then repeat with the remaining third. Turn the dough 90 degrees and fold it over again into thirds. Place the dough back into the oiled container and continue to bulk prove for a further 1 hour.
Use a blunt knife or divider to divide the dough into two even-sized portions, weighing about 500 g (1 lb 2 oz) each. Working with one portion of dough at a time, continue to shape the loaves following the instructions for shaping a round loaf.
Line two small baskets with a tea towel (dish towel) in each, lightly dust both with flour and place the dough inside, seam side up, so that when you go to turn the loaves out onto trays it is with the seam down onto the tray. If you are using a cane basket, you don’t need the tea towel and can simply dust the basket with flour. Alternatively, you can place the loaves on a baking tray lined with baking paper, seam side down. Place in the refrigerator loosely covered with a plastic bag for 8–12 hours.
Preheat the oven to its highest temperature. Remove the loaves from the refrigerator and let them rest, covered, in a humid place (25°C/77°F) — this could take anywhere between 1 and 4 hours — until each loaf has grown in size by two-thirds. If the loaves push back steadily and quickly when you push lightly into them with a finger then they are ready to bake. Score the loaves and place in the oven.
Spray the oven with water and reduce the temperature to 220°C (425°F/Gas 7). Bake the loaves for 20 minutes, then turn the loaves or trays around and bake for a further 10 minutes, watching carefully to make sure that the loaves do not burn. Check the base of each loaf with a tap of your finger — if it sounds hollow, it is ready. Baking should take no longer than 40 minutes in total.