Mix the flour, yeast, salt and bread spices and knead in the sourdough well. Then gradually pour in lukewarm water until you have a smooth, cracking yeast dough. Knead the dough vigorously for 10 to 15 minutes, this will loosen the bread. Cover and let the dough rise in a warm place for 1 to 2 hours. Meanwhile, peel and core the apple and cut into small cubes. If you like, you can shred the walnut halves even further, I`ll leave them whole. After the resting time, knead in the apple cubes and the walnut halves by hand, form an elongated loaf of bread and cut lightly at the top. Rub generously with water to make the crumb more crispy. Preheat the oven to 250 ° C and bake the bread in it for about 5 minutes. Then turn the temperature down to 180 ° C and bake for another 45 to 50 minutes. The bread is ready when it sounds hollow when you tap the underside.