Prepare leeks by thinly slicing the white and light green parts. Submerge slices in water and pull into rings. Rinse well to remove any grit and drain well. Put broth in medium pan and heat. In large skillet, heat olive oil. Add leeks, garlic, onion and mushrooms, if using. Saute for 3 minutes. Add rice to mixture and cook 1 minute. Put one ladle of broth in the rice mixture and stir until completely absorbed. Add another ladle and repeat process until the rice is done and broth is absorbed. If you need more broth, you can substitute hot water. Add butter, lemon juice and zest and half of the parmesan and stir until butter is melted. Plate and sprinkle with additional parmesan. This can be a main dish or a side.