Dissolve 2 cubes fresh yeast in 150 ml lukewarm water, add 1 teaspoon sugar, and stir. Add this mixture to 900 g wheat flour and 100 g rye flour in a food processor. Mix for 2 minutes.
Add 250 ml lukewarm water, 250 ml lukewarm buttermilk, 1½ tablespoon salt, and 20 g warmed lard. Knead for 8-10 minutes until smooth and elastic. Cover and let rise for 45 minutes until dough has doubled in size.
Knead for 2-3 minutes on a floured work surface. Shape into a long loaf, wrap the end in a floured cloth, and let rise for 15 minutes.
Place loaf end-up on a greased baking sheet. Cut lengthwise down the center with a sharp knife. Let rest for 10 minutes.
Bake for 40 minutes in an oven preheated to 225°C (437°F). Pour a cup of hot water into the drip pan to create steam. Cool before serving.