Use dark rye flour, which has more rye flavor than white rye flour. Also, please read About Starters or Pre-Ferments.

Combine in a medium bowl and let stand until the yeast is dissolved, about 5 minutes:

Add:

Stir rapidly with a wooden spoon until you notice elastic strands pulling away from the sides of the bowl, about 2 minutes. Cover the bowl tightly with plastic wrap and let rise at room temperature until bubbling and tripled in volume, about 6 hours; or let the starter rise for about 14 hours in the refrigerator.

Scrape the sponge into a large bowl or a stand mixer fitted with the paddle attachment and add:

Stir rapidly with a wooden spoon or mix on medium-low speed until the dough just comes together. Adjust the consistency of the dough by adding more bread flour or water. Switch to the dough hook. Work in by hand or with the mixer for 2 minutes:

Knead by hand or with the mixer on medium-low speed until the dough is smooth, elastic, and firm, 7 to 10 minutes. Transfer the dough to an oiled bowl and turn to coat with oil. Cover the bowl tightly with plastic wrap and let rise in a warm place (75° to 85°F) for 1 ½ hours. The dough will rise only very slightly; do not leave it longer, or it will overferment the starter, which will make the bread heavy (yeast cells eat rye flour very quickly).

Grease a large baking sheet and sprinkle with:

Divide the dough in half and shape each half into a round loaf. Place the loaves on the baking sheets, leaving space between them. Cover with oiled plastic wrap and let rise in a warm place (75° to 85°F) for 1 ½ hours.

Position racks in the center and lower third of the oven. Place a metal baking pan or cast-iron skillet on the lower rack. Preheat the oven to 450°F.

Place the loaves on the center rack. Immediately pour 2 cups boiling water into the pan or skillet on the lower rack. Bake until well browned, the loaves sound hollow when tapped on the bottom, and the internal temperature reaches 105° to 110°F, about 45 minutes. To further set the crust, turn the oven off and let the loaves remain in the oven for 10 minutes. Let cool completely on a rack.