In a large bowl combine the rye flour, sourdough starter, salt, German bread spice and water. Mix and knead the ingredients by hand until evenly incorporated; cover and allow the dough to rest until it has increased noticeably in volume — at least 1 hour, preferably longer or overnight for best flavor and digestibility.

Turn the dough out onto a well-floured work surface. Fold and coax the dough inward from the edges to form a tight, compact round, working gently to avoid trapping excessive air. This shaping helps produce an even crumb and prevents large holes.

Place the shaped loaf seam-side up in a proving basket dusted with flour. Cover and let it proof at room temperature for about 1 hour, or until the loaf appears puffed and roughly doubled.

While the loaf finishes its final rise, preheat the oven to 220°C (428°F) with your baking stone, Dutch oven, or cast-iron surface inside. Put a heatproof dish (I use a small cast iron skillet) on the bottom rack to create steam during baking.

Carefully invert the proved dough onto a piece of parchment and transfer it to the preheated baking surface. Pour about 1 cup of water into the heatproof vessel on the bottom rack to generate steam, then promptly close the oven.

Bake for roughly 1 hour total. After the first 20 minutes reduce the temperature to 180°C (350°F) and continue baking for another 35–40 minutes, or until the loaf sounds hollow when tapped on the bottom and has a well-browned crust.

Remove the bread from the oven and cool completely on a wire rack before slicing to allow the crumb to set and flavors to mature.