Preheat the oven to 350°F Cut the squash into quarters and remove the seeds.
Dice the flesh into 1in cubes and toss with rosemary and olive oil.
Place in a roasting tin and roast for 20 minutes or so, until soft and slightly caramelised.
Heat the chicken stock and keep warm.
Melt half of the butter in a large frying pan over a low heat and fry the onion and celery for 15-20 minutes, until very soft.
Remove the pan briefly from the heat, add the garlic and rice and stir until the grains are coated with butter.
Return the pan to a high heat - add 2 ladlefuls of stock and simmer until the rice has absorbed it all, stirring all the time.
Add the roasted squash to the rice and simmer until it softens and breaks up into the risotto.
Repeat the process of adding the stock, 2 ladles at a time, until the rice has absorbed nearly all the liquid - this should take about 15 minutes.
Stir in the remaining butter and Parmesan cheese. Stir and season with salt and freshly ground black pepper. Garnish with pumpkin seeds and serve.