Using a stand mixer, combine the flour, salt, rosemary and yeast in the mixing bowl. Add the water and oil to the poolish, stir to loosen it, and pour it into the flour mixture. Using the dough hook, mix on low speed until a rough dough forms. Cover and let it rest for 10 minutes.
After the dough has rested, add the olives and continue to mix until it is very smooth (about 5 minutes).
Take the dough out of the mixing bowl onto a floured surface, flatten it out with your hands, and sprinkle it with the walnuts. Knead the bread by hand until all of the walnuts are incorporated.
Wash and dry your mixing bowl. Rub it with olive oil, place your bread dough in, and cover with plastic wrap or a damp cloth. Let it ferment until light and doubled in bulk, about 6 hours.
After the dough has risen, punch it down and divide into two equal portions. Knead each ball a bit more, and then cup your hands around the dough and rotate it in circles until a smooth ball forms. Place your dough balls onto an oiled cookie sheet. Allow to rise for about 1 hour; preheat the oven to 400°F about 30 minutes into this rising.
Slash the loaves 3-4 times with a serrated knife before you put them in the oven, and allow to bake for 30-40 minutes, or until well-browned and hollow sounding when you knock on the bottom. Allow to cool for 20-30 minutes on a wire rack before slicing.