Generously grease a 900g/ 2lb loaf tin with butter (or oil but I find butter works best).
Place a mixing bowl onto the scales and weigh 420 g tepid water and 75 g of sourdough starter.
Mix the starter and water with a spatula (don’t worry about being thorough).
Add the rye flour, strong malted bread flour, mixed seeds and salt in this order.
Mix well with a spatula until all the flour is incorporated. It should be the consistency of wet concrete.
Scrape the dough into the loaf tin.
Level the surface with a wetted rubber spatula or hand to eradicate any air pockets. Aim for the sides being lower than the middle (which should be slightly raised).
Place the loaf tin into a large clear bag, seal with a clip and leave on your counter to bulk ferment until the dough has risen by 25% and has around 10 pinprick holes. This can take between 5 and 16 hours but will depend on the room temperature and time of year. (See notes).
Brush a little water over the top of the rye sourdough, then sprinkle with 35 g of mixed seeds.
Place the loaf tin into the slow cooker and bake on high for 3 hours and 10 minutes (core temperature should reach 98 – 99°C). Alternatively, bake in a preheated oven on a baking tray on the lowest shelf at 175℃, 155℃ fan, 347℉, gas mark 4 for 1 hour and 15 minutes.
Once baked, allow the rye bread to cool in the tin for 10 minutes.
Then loosen with a nonstick spatula if necessary and tip the rye bread onto a wire rack to cool.
Once cool put the loaf into a bag to soften (it will be hard and crusty at first). Allow 12 to 24 hours to pass before slicing and eating the rye bread.
Store the loaf in a sealed bag or beeswax wrap to maintain moisture. Alternatively slice up and freeze in a food bag. Frozen Danish rye bread makes great toast.