Roast the squash: Preheat the oven to 425°F (220°C) and adjust the rack to the center. On a rimmed baking sheet, toss the cubed butternut squash with 2 tablespoons olive oil, ½ teaspoon salt, pepper, and pumpkin pie spice. Spread in an even layer and roast for 15-20 minutes until golden and tender, stirring halfway through. Then lower the heat to 350°F.
Start the risotto: Over low heat, add the chicken broth to a 4-quart saucepan to warm. In a Dutch oven over medium heat, add 1 tablespoon olive oil, onion, and ¼ teaspoon salt. Sauté the onion until soft and translucent, about 8-10 minutes. Add chopped garlic and stir until fragrant, about 1 minute. Stir in Arborio rice and toast for 1–2 minutes until slightly opaque. Pour in 3 cups of the warmed chicken broth, stir, cover tightly with a lid, and bake at 350°F (175°C) for 30 minutes.
Make the butternut purée: In a personal blender, blend 1 cup of roasted squash with 1 cup of warmed chicken broth until smooth and silky, about 30 seconds. Set aside.
Finish the Risotto: Remove the Dutch oven from the oven and add to the stove over low heat. Get ready to stir! Add 1 cup warm broth, sherry vinegar, squash purée, parmesan, and butter. Stir constantly for 3-4 minutes until creamy and smooth. Taste and adjust seasoning as needed (and make sure the rice is fully cooked). If it is still undercooked, add more broth and continue to stir until cooked through. Finally, stir in the remaining roasted butternut squash. If you want a richer risotto, stir in ¼ cup of heavy cream or mascarpone cheese at the end. Keep warm while you make the brown butter.
Make the brown butter: In a small skillet, melt the butter over medium heat. Add the sage leaves and pine nuts. Stir and swirl the pan frequently until the butter foams and turns a deep golden brown, and the sage leaves crisp up, about 3-4 minutes. Remove from heat immediately to prevent burning.
Serve: Spoon the risotto into bowls, drizzle with the brown butter sauce (making sure to get some sage and pine nuts in each bowl). Garnish with extra parmesan and thyme or parsley for color if desired.