In a large glass bowl, whisk rye sourdough starter with water until dissolved and frothy.
Add flour and mix with a spoon until no dry bits remain. The mixture will be sloppy.
Cover tightly, and leave at room temperature until it becomes bubbly and ‘soft’ looking (bubbles are easier to see under the surface, that’s why glass bowl is convenient). This can take anywhere from 4 to 12 hours, depending on the temperature but typically rye ferments faster that wheat because it has more nutrients that fermentative yeast and bacteria like. I often start late in the evening and make the main dough early morning.
MAKE MAIN DOUGH
Mix starter dough with the main dough ingredients until combined and uniform in color.
Cover, and leave at room temperature for 4-10 hours. Here you want to watch the dough more than the time, look for it to soften and puff up (it won’t rise much), then drop slightly. If you don’t have a chance to watch, just go with 6 hours.
With a spoon or a stiff spatula, mix the dough well, and transfer to a bread pan lined with parchment paper (don’t skip the paper even with non-stick pans). I use a deep Pullman loaf pan because I like the straight up sides, but this amount of dough doesn’t fill it all the way even after baking. Regular loaf pan would work just fine. Smooth the top with a wet hand.
Let it rest in the pan no longer than 30 minutes, but no longer than that – if you allow it to rise in the pan too long, it will collapse during baking, which will make the crumb brick-dense and gooey.
Bake in a pre-heated oven at 350ºF (180ºC) for 45-50 minutes. It’s best to wait for 24 hours before cutting into it, I know it’s hard but try to wait for at least 8 or 10 hours.
Notes
This amount of dough makes a fairly small loaf, about the size of a classic pumpernickel or just slightly bigger. You can increase the amount of dough for a larger, more sandwich-like bread.
Starter dough is the same as what bakers call 'leaven', or 'levain'. The purpose of it (as opposed to mixing a couple of spoonfuls of starter together with all the ingredients is to have a fair amount of very active bread yeast that can be incorporated into the rest of the ingredients evenly. This allows for uniform fermentation to get a nice stable crumb.
You can add any spices you like, and add more cacao powder (I use up to ½ cup sometimes, just for fun). I like adding cardamom. Whole coriander seeds are used in this type of bread in Russia.