The night before, place the Whole rye kernels and Cracked rye (rye chops) in a large bowl and pour over enough just-boiled Water to cover them by a couple of centimeters; stir, cover and leave to soak at room temperature for 8–12 hours so the grains swell and soften.
In a separate large mixing bowl combine the Rye sourdough starter with a little warm Water to loosen it, then stir in the Wholemeal rye flour until you have a thick, shaggy batter.
Add the soaked grain mixture (including any remaining soaking liquid) to the batter, then fold in the Sunflower seeds and Flaxseeds along with the Dark syrup and Sea salt; the dough will be very sticky and dense—this is normal for rugbrød.
Adjust the consistency with a splash more Water if the mixture feels unworkably dry; aim for a wet, scoopable dough rather than a conventional elastic loaf.
Cover the bowl with a cloth or plastic and let the dough ferment at room temperature for 3–6 hours, or until it shows some gentle rising and a slightly domed surface. Longer, cooler fermentation will deepen the flavor.
Grease and line a 1–2 kg loaf tin (or two smaller tins). Transfer the dough into the tin, smooth the top with a wet spatula and cover loosely. Allow a final rest of 30–60 minutes for a short proof.
Preheat the oven to 200°C (390°F). Place the loaf on the middle rack and bake covered (with foil or a lid) for 45–60 minutes to keep the interior moist, then remove the cover and bake a further 20–30 minutes until the crust is firm and a skewer inserted into the center comes out mostly clean.
Turn the loaf out of the tin and cool completely on a wire rack before slicing—rugbrød slices much cleaner once fully cooled and the crumb has set. The flavor will improve after resting a day; store wrapped at room temperature for several days or freeze slices for longer keeping.
