Prep the Squash: Roast diced squash with 1 tbsp olive oil, salt, and pepper at 200°C / 400°F for 20 minutes (or sauté in a pan until soft). Mash half for creaminess and keep half in cubes for texture.
Start the Base: Heat 2 tbsp olive oil in a heavy pot. Sauté onion and garlic for 2–3 minutes until fragrant.
Toast the Rice (Tostatura): Add Arborio rice. Stir for 1–2 minutes until the edges look translucent.
Add Broth Slowly: Pour in 1 ladle of hot stock. Stir gently until absorbed. Continue adding broth, 1 ladle at a time, stirring often. Keep heat at medium-low for steady simmer.
Fold in Squash: Halfway through cooking (about 10 minutes), stir in mashed squash. Keep adding stock until rice is tender but al dente, about 18–20 minutes total.
Finish (Mantecatura): Remove from heat. Stir in butter, Parmesan, lemon zest, and nutmeg. Adjust with a splash of stock if too thick.
Serve: Plate the risotto, top with roasted squash cubes, fresh sage, and extra Parmesan. Serve immediately.