Make the levain: in a medium glass bowl, mix the ripe sourdough starter, 200g rye flour, and 200 g filtered water. Mix thoroughly, cover lightly, and leave on counter to ferment for 12 hours. Use a big enough bowl: this levain gets bubbly!

After 12 hours, check your levain for readiness. It should be very bubbly throughout with a mild sour scent.

In a large mixing bowl, mix together the ripe levain, 175 g rye flour, 375 g white bread flour, salt, and 356 g filtered water.

This dough will be sticky! Don't add more flour: you will wind up with dry bread if you do. Make sure you scrape the sides of the bowl.

Once mixed, take a wooden spoon and use the back if it to work the dough a bit.

Lightly cover the mixing bowl and leave on the counter for bulk fermentation.

Set a timer for one hour into bulk fermentation and perform one thorough set of stretch and folds. You might want to lightly oil your hands first.

Re-cover dough and leave to sit for the remainder of bulk fermentation, three more hours.

At the end of bulk fermentation, the dough should have risen substantially and be quite bubbly. Don't intentionally deflate the dough, it will happen naturally when you turn it out.

Lightly oil a clean work surface and turn the dough out onto it. Divide the dough in half.

Pre-shape the dough: use a bench scraper and your hands to lightly form each dough into a round. Leave on the counter to rest for about 30 minutes.

Prep a baking sheet(s) by lining with parchment paper. Use a bench scraper to form the dough more firmly into rounds, then transfer to the prepared baking sheets. I place them gathered-side-down so that the top is smooth. Typically, the loaves can share a baking sheet, if you want.

Lightly cover the loaves with plastic wrap and leave on the counter at warm room temp to proof for 1.5 hours.

Near the end of the proofing time, place an oven rack in the middle of the oven (leaving enough room below for a pan) and preheat the oven to 425 degrees.

Place baking sheet in the oven and set a timer for 20 minutes. When the timer goes off, decrease oven temp to 350 degrees and remove steaming pan. Bake for 15 more minutes.

Remove finished bread to a cooling rack and allow to cool COMPLETELY (at least 2 hours) before slicing. It will be gummy if you slice early.