Make the Preferment: Combine the 4 grams salt, 195 grams lukewarm water, 90 grams dark rye flour, 100 grams all-purpose flour, and 40 grams sourdough starter in a medium bowl using a rubber spatula or spoon. Cover airtight and leave at room temperature for 10 to 12 hours.
Make the Sponge: About 30 minutes before mixing the final dough, combine the 200 grams lukewarm water and 260 grams all-purpose flour in a small bowl. Mix by hand until a smooth dough forms. Cover airtight and let rest at room temperature for 30 minutes.
Prepare the Onion Variation: If making Zwiebelbrot, trim, peel, and slice the large yellow onion. Cook it with the 14 grams butter over medium to medium-low heat, stirring occasionally, for about 10 to 15 minutes, or until golden brown and most of the moisture has cooked away. Avoid adding overly wet onions to the dough, as excess moisture can affect the dough consistency.
Combine the Dough: Do not premix the main dough ingredients separately. Add the 70 grams lukewarm water, 9 grams salt, 175 grams dark rye flour, and 3 grams active dry yeast directly to the prepared preferment and sponge. Add the cooked onions at this stage if making the onion version.
Knead the Dough: Knead by hand or use a stand mixer fitted with a dough hook for about 5 minutes, until all ingredients are fully incorporated and the dough is homogeneous and only slightly sticky.
First Rise: Cover the dough airtight and let it rest at room temperature for 1 hour. By the end of this rise, the dough should have increased in size by approximately half.
Shape the Loaf: Shape the dough into either a round boule or a long batard, depending on the shape of your proofing basket. Fold sections of the dough toward the center, pressing and working the folded edges into the middle until the loaf holds its desired shape.
Prepare the Proofing Basket: Dust the proofing basket generously with white flour. Place the shaped loaf into the basket seam side down. Cover with a linen towel or the basket liner.
Final Proof: Let the shaped dough rise at room temperature for 45 minutes.
Preheat the Oven: Preheat the oven to 250°C (480°F). Place a casserole dish filled with hot water at the bottom of the oven to create steam.
Prepare for Baking: Line a baking sheet with parchment paper. Carefully invert the proofed dough from the basket onto the prepared baking sheet. Check that there is still plenty of water in the casserole dish before transferring the bread to the oven.
Score the Loaf, If Desired: Optionally score the surface of the dough to control where the loaf opens during baking. Scoring is not required.
Bake the Bread: Transfer the loaf quickly to the hot, steamy oven and bake for a total of 60 minutes, following the source baking method.
