Heat the stock until almost boiling and then keep over a very low heat. In a separate heavy-based saucepan fry the onion in the oil over a low heat until soft but not browned. Add the chopped sage and cook for a couple more minutes.
Add the rice and mix well for a few seconds to coat the grains with oil, then pour in one-third of the stock and bring to a gentle simmer. Cook until almost all the stock is absorbed. Add the pumpkin or squash and a little more stock, and continue to simmer gently until the stock is absorbed.
From then on add more stock a little at a time, until the pumpkin is soft and the rice nicely al dente (has a little bite to it). You may not need all the stock, but the texture should be loose and creamy.
When the risotto is almost ready, heat the sunflower oil in a small pan and quickly fry the sage leaves until crispy - it only takes a matter of seconds.
Stir the butter into the risotto, and season well with salt and pepper. Divide into 4 bowls and sprinkle a few crispy sage leaves over each portion. Bring the cheese and a grater to the table for your guests to serve themselves.