Mix 120 g water, 160 g sourdough discard, and 130 g dark rye flour by hand until fully combined

Cover and let ferment at 28–30°C (82–86°F) for 3–4 hours, until doubled or even tripled in volume; the mixture should be very loose and airy

Add 230 g water, 10 g salt, 30 g sugar, 300 g bread flour, and 150 g rye flour

Knead until the dough comes together into a smooth ball and clears the sides of the bowl

Round the dough, lightly oil the bowl to prevent sticking, place the dough inside, and cover

Let rest for 3–4 hours, until the dough has increased to about 2 to 2½ times in volume

Shape into a tight round loaf and place seam-side up into a floured banneton

Proof for 30–40 minutes, until slightly puffed and visibly larger

Preheat oven to 500°F (260°C) with a baking steel inside

Transfer the loaf onto parchment paper, slide onto the hot baking steel, and immediately cover with a cold baking shell

Bake for 10 minutes at 500°F (260°C), then remove the baking shell

Lower oven temperature to 450°F (230°C) and bake for 10 minutes

Lower temperature to 350°F (175°C) and bake for an additional 20–25 minutes, until deeply colored and fully baked

Remove from oven and let cool completely before slicing