Preheat the oven to 350°. Spread the walnuts on a baking sheet and toast for about 8 minutes, or until deeply browned. Let cool.
In a large bowl, sprinkle the yeast over the water and let stand until foamy. Stir in the melted butter, egg and salt. Add the all-purpose and whole wheat flours and the walnuts, stirring until the dough becomes stiff. Scrape the dough onto a lightly floured surface and knead until a smooth dough forms. Transfer the dough to a lightly oiled large bowl, cover with plastic wrap and let stand in a warm place until doubled in bulk, about 1 1/2 hours.
Punch the dough down and cut it in half. Knead each half briefly and shape into 6-inch rounds. Set the rounds on a large, lightly oiled baking sheet about 4 inches apart. Let stand until the loaves have almost doubled in volume and hold an impression when lightly pressed, about 30 minutes.
Preheat the oven to 375°. Bake in the center of the oven for about 50 minutes, or until the loaves are nicely browned and sound hollow when tapped on the bottom. Transfer the loaves to a rack to cool completely before slicing.