Heat oil and 1 tbsp butter in a wide pan over medium heat. Add onion and cook until soft, 4–5 minutes.
Stir in rice and cook 1–2 minutes until edges look translucent. Pour in wine if using and let it absorb.
Add warm broth, one ladle at a time, stirring so rice absorbs liquid before adding more. This takes 18–20 minutes.
When rice is tender, stir in mashed squash, remaining butter, sage, and Parmesan. Season with salt and pepper.
Cool completely before freezing in portions. Reheat gently with a splash of broth.