In a large deep pan, heat butter and olive oil over medium heat. Add onion and cook for 2 to 3 minutes until soft. Add garlic and cook for 30 seconds until fragrant.
Add arborio rice and stir for 1 minute so it is well coated in the butter and oil.
Pour in white wine and cook, stirring, until it has mostly absorbed into the rice.
Add warm stock, 1 cup at a time, stirring gently. Let each addition absorb before adding the next.
Continue adding stock, stirring between each addition, until the rice is tender with a slight bite and the texture is creamy. With the final addition of stock, add the roasted pumpkin chunks, gently stirring and pressing some into the rice to create a creamy texture. This will take around 20 to 25 minutes.
Stir through parmesan. Remove the pan from heat, then add sour cream and stir until smooth and creamy. Taste and season with salt and pepper if required.
Spoon risotto into bowls. Top each bowl with roasted pumpkin wedge, some crispy sage, toasted pepitas, and extra parmesan before serving. Drizzle with some olive oil and some cracked black pepper. Serve.