Grate a handful of Parmigiano-Reggiano and using a vegetable peeler, make shavings with the rest. Set aside. In a large frying pan, heat some oil and cook celery, carrots and onions until soft. Add the spinach. Add the rice and stir well to coat with the vegetable mixture, cooking the rice for several minutes. Pour in the broth a little bit at a time, stirring constantly, until the rice is cooked. Take the rice off the stove, add the butter and stir until melted. Finally, stir in the grated Parmigiano-Reggiano and serve with cheese shavings sprinkled over each portion.