Combine 1 cup white flour and wheat flour, walnuts, sugar, yeast and salt. Heat water, yogurt, and butter to 120 degrees; add to dry ingredients; beat until smooth. Stir in more flour to form a soft dough. Turn on a floured surface and knead until smooth and elastic-4 minutes. Shape into ball, cover for 20 minutes. Bake 400 degrees for 25-30 minutes.