1. Preheat oven to 400°. Place all squash cubes on an aluminum foil-lined rimmed baking sheet. Drizzle with 1 tablespoon olive oil and vinegar, and toss. Season with 1 tablespoon thyme, sea salt, and pepper, and roast 35 to 40 minutes or until squash is tender and browned, turning once. Remove from oven, and set aside.
2. Bring broth to a boil in a 3-quart saucepan over medium-high heat. Turn off heat, cover, and keep warm.
3. Heat remaining olive oil and butter in a large sauté pan over medium heat. Add onion, and sauté, stirring frequently, 2 minutes or until tender and translucent. Add garlic, and sauté, stirring constantly, 1 minute more.
4. Add rice, and cook, stirring constantly, 2 minutes or until grains are well coated and the outsides turn translucent. (Do not let rice brown.) Add white wine, and cook 30 seconds or until most of it evaporates.
5. Add 1 cup chicken broth, and stir constantly until mostly absorbed. Add additional broth in 1-cup increments, stirring after each addition until most of the broth is absorbed. Stop adding broth when rice appears to be saturated. (You may not need all 5 cups broth.) Cook, stirring frequently, 20 to 25 minutes or until rice is tender and creamy but not mushy.
6. Stir in cooked squash and any collected juices, and add remaining 1 teaspoon thyme. Reduce heat to low, and cook 2 minutes or until squash is thoroughly warmed. Add a little more chicken broth if the risotto gets too dry. Remove from heat, and stir in 1/4 cup Parmesan. Season with additional sea salt and pepper, if desired. Sprinkle with remaining Parmesan, and serve warm.
Kitchen Confidential: Sara's Secrets for Perfect Risotto:
The spoon: A regular spoon will work, but a risotto spoon will get the tender results you're looking for. Rice passes through the spoon's hole instead of getting mashed.
The rice: Once the rice turns glossy and translucent, it is fully coated with olive oil and butter and ready for the liquid.
The broth: Keep it hot! For maximum absorption and creamy texture, make sure the chicken broth is still hot when it's added to the risotto.