Sauté onions in olive oil, add rice, stir until covered in oil, add boiling chicken stock, and continuously stir until stock is absorbed. Continue to add stock until rice is creamy. Take from the plate and add parmesan and butter. Stir, if required add some stock or water until all of the risotto is creamy. Season.
Just before the rice is cooked you can add finely diced vegetable cubes or peeled potatoes.