For the sponge starter, mix together 1/2 tsp active dry yeast, 1/2 cup lukewarm water and add 3/4 cup of bread flour.
Stir rapidly with a wooden spoon until the batter starts pulling away from the sides of the bowl in elastic strands, about 1-2 minutes.
Cover the bowl and allow it to sit at room temperature for about 6 hours. The sponge will triple in volume.
To make the bread:
In a large bowl or in the bowl of a stand mixer, place: 3.5 cups rye flour, 3.5 cups bread flour, 2.5 cups water and sponge starter
Mix together and then knead by hand for about 7-8 minutes or on medium low-speed for the same time. The dough will be smooth.
Transfer it to an oiled bowl and turn over once to coat with the oil.
Cover and let the bread rise in a warm place for exactly 1.5 hours. Don't let it stand any longer because the loaf will get too dense and heavy.
Divide the dough into two, and shape each ball into an oval by pulling on the sides and tucking them under. Place the loaves next to each other, but with a few inches between them, on a large, oiled baking sheet sprinkled with cornmeal.
Cover loosely with a napkin and let the loaves rise in a warm place for 1.5 hours.
Half an hour before the loaves are ready for the oven, preheat the oven to 450 C.
A minute before you put the loaves into the oven, spritz the walls of the oven with water using a spray bottle.
Quickly score a cross into the top of each loaf using a very sharp blade. It's important to use a quick, single motion so as to not deflate the loaves.
Bake the loaves in the oven for 45 minutes or until such time as the bottom of each loaf sounds hollow when rapped.