In a bowl, stir ½ cup rye flour, ¼ cup water, and yeast until a smooth paste forms. Cover with plastic wrap, and let sit for 24 hours. Repeat this process for the next eight days, adding the same amounts of rye flour and water, to make starter (which you can keep alive, in the fridge, by adding the same amounts once weekly.)
On 10th day, place ¼ cup starter in a bowl. Stir in ½ cup rye flour and ⅓ cup water until smooth to create sourdough culture for this loaf. Let sit for 8 to 24 hours.
Uncover culture, and add remaining 1 cup rye flour and ½ cup water, along with bread flour, salt, and half the seed mix (Figure A). Stir until dough forms (Figure B); let dough sit for 20 minutes. Transfer to a floured work surface; knead until smooth and elastic, about 10 minutes (Figure C). Transfer to a greased bowl; cover with plastic wrap. Place in a cold oven; let sit until slightly inflated, about 1 hour. Transfer to a work surface and flatten. Fold top and bottom edges toward middle. Return dough to bowl, seam side down; cover, and return to oven. Let sit until doubled in size, about 3 hours.
Transfer dough to surface. Positioning your hands on outside edge of dough, rotate dough over surface to form a taut dome, pinching edges underneath. Transfer, seam side up, to a floured kitchen towel in a colander. Cover loosely with plastic wrap; let sit until doubled in size, about 3 hours. One hour before baking, place a cast–iron skillet on bottom rack of oven; position another rack above skillet; place a baking stone on top of it. Heat to 400°.
Invert dough onto a parchment paper sheet on a rimless baking sheet. Spray with water, and cover with remaining seed mix (Figure D). Using a razor, slash a hash tag pattern in top of dough. Using paper, slide loaf onto stone. Place ice in skillet. Bake until dark brown, about 1 hour; let cool before serving.