Preheat oven to 180°C. Line a baking tray with baking paper. Place the pumpkin on the tray and drizzle with 1 tsp of oil. Toss to combine. Bake for 25-30 mins or until pumpkin is tender.
Meanwhile, bring stock to the boil in a saucepan over medium heat. Reduce heat to low and keep at a gentle simmer.
Heat 2 tsp of oil in a large saucepan over medium heat. Add the celery and onion and cook, stirring, for 5 mins or until onion softens. Add the rice and cook, stirring, for 2 mins or until the grains appear glassy. Add the wine and cook, stirring constantly, for 2-3 mins or until the wine is absorbed. Add ½ cup (125ml) of the simmering stock to the rice mixture, stirring constantly with a wooden spoon until the liquid is completely absorbed. Continue adding ½ cupfuls of stock, stirring after each addition until the liquid is absorbed, for 20 mins or until the rice is tender yet firm to the bite and the risotto is creamy.
While the risotto is cooking, heat remaining oil in a large frying pan over medium heat. Season chicken and add to the pan. Cook for 5 mins each side or until cooked through. Transfer to a plate and thickly slice. Melt butter in the frying pan over medium heat. Add sage leaves and cook for 3 mins or until butter is browned and sage is crisp.
Add the pumpkin and parmesan to the rice mixture. Stir to combine. Divide the risotto among serving plates. Top with chicken and butter mixture.