Make the rye bread sponge

Cup of Yum
1. In the bowl of a stand mixer, mix the sponge ingredients by hand until incorporated. Cover and let ferment at room temperature (68 to 72°F or 20 to 22°C) overnight or for 10 to 12 hours. The sponge will be very bubbly, have a clean sour smell, and will have doubled in volume.

Make the rye bread dough

1. Add the water and 1.8 ounces (50 grams) bread flour to the sponge and mix by hand until it forms a slurry, 1 to 1 1/2 minutes. Add 8.8 ounces (250 grams) bread flour and the rye flour and use the paddle to mix at low speed until the dough is evenly hydrated, 2 to 4 minutes. Cover the dough and let it rest at room temperature for 15 to 20 minutes.
2. Add the salt and yeast to the bowl with the dough and mix with the dough hook of your stand mixer on low speed until the gluten is very well developed, 30 to 35 minutes. The dough should be sticky and very stretchy. Cover and let ferment at room temperature until doubled in volume, 2 to 2 1/2 hours.
3. Preheat the oven to 430°F (220°C) with an oven rack adjusted to the middle position and a second rack in the lower position with a steam pan on it. [Editor’s Note: A steam pan is simply a pan filled with some water. You’ll want to use a pan with relatively high sides and only fill it partway with water as you’ll need to remove the pan from the oven when the water is very hot and you don’t want it sloshing all over you or your oven.] If using a baking stone, place it on the middle rack.
4. Turn the dough onto a generously floured work surface and gently form it into a ball by folding it toward you, rotating 90 degrees, and repeating until the bottom surface is smooth. Flip the dough. If a traditional Galician rye bread shape is desired, pinch a golf ball-size piece of dough from the center of the loaf and carefully stretch it upward without separating it from the loaf, then form a depression in the dough and nestle the ball of dough inside the depression. Place the dough on a parchment-lined baking sheet or, if using a baking stone, on a well-floured peel. Cover and proof at room temperature for 10 minutes.

Bake the rye bread

1. Bake the rye bread for 15 minutes.
2. Remove the steam pan from the oven, quickly shut the oven door, and lower the temperature to 390°F (200°C). Continue to bake the bread until the crust is golden brown, the loaf thumps when tapped with a finger, and the internal temperature is at least 198°F (92°C), 15 to 25 minutes more.
3. Transfer the loaf to a wire rack and let it cool completely before slicing.