1For the sourdough, dough may be prepared the day before.
2Place flour in a large bowl.
3Mix yeast water in a small bowl and pour over the flour.
4Mix until a firm dough forms.
5Place the sourdough on a lightly floured surface and knead for 8-10 minutes.
6Place in a lightly greased bowl, cover with plastic wrap, and let ferment in a warm area for 3 hours.
7Place dough on lightly floured work surface and punch to remove air.
8Place in a bowl covered with plastic wrap or in a sealed container.
9Refrigerate dough overnight, or let it ferment three more hours and remove two hours before making the bread.
10For the base dough, mix white and whole rye flours in a bowl and make a well in the center.
11In a small bowl, combine water and yeast and let stand 5 minutes until liquid yeast forms.
12Pour liquid yeast into the flour mixture, add salt and sourdough.
13Knead well until dough is smooth and elastic.
14Form a ball and let rise for 1 hour.
15After that, knead well to remove air.
16Form two round loaves and sprinkle with flour.
17With a small, sharp knife, make four shallow cuts on bread surface, forming a cube.
18Set both loaves on a floured baking sheet, cover, and let rise for 1 hour.
19Preheat oven to 240°C (approximately 460°F).
20Bake on the upper oven rack.
21Spray oven with water and bake for 30 minutes until golden brown. Cool on a rack.