Heat 2 tablespoons oil in 12-inch nonstick skillet over medium-high heat until shimmering but not smoking. Add about 3 1/2 cups squash in even layer and cook without stirring until golden brown, 4 to 5 minutes; stir in 1/4 teaspoon salt and 1/4 teaspoon pepper. Continue to cook, stirring occasionally, until squash is tender and browned, about 5 minutes longer. Transfer squash to bowl and set aside.
Return skillet to medium heat; add reserved squash fibers and seeds and any leftover diced squash. Cook, stirring frequently to break up fibers, until lightly browned, about 4 minutes. Transfer to large saucepan and add vegetable broth and water; cover saucepan and bring mixture to simmer over high heat, then reduce heat to medium-low to maintain bare simmer.
While broth mixture is simmering, add 1 teaspoon olive oil to now-empty skillet and swirl to coat. Add 4 ounces baby spinach and cook, covered, over medium heat, until leaves begin to wilt, about 2 minutes. Uncover and cook, stirring constantly, until fully wilted, about 30 seconds. Set aside.
Melt 3 tablespoons butter (or margarine) in now-empty skillet over medium heat; when foaming subsides, add onions, garlic, remaining 1/2 teaspoon salt, and remaining 1/2 teaspoon pepper. Cook, stirring occasionally, until onions are softened, 4 to 5 minutes. Add rice to skillet and cook, stirring frequently, until grains are translucent around edges, about 3 minutes. Add wine and cook, stirring frequently, until fully absorbed, 4 to 5 minutes.
Meanwhile, strain hot broth through fine-mesh strainer into medium bowl, pressing on solids to extract as much liquid as possible. Return strained broth to saucepan and discard solids in strainer; cover saucepan and set over low heat to keep broth hot.
When wine is fully absorbed, add 3 cups hot broth and half of reserved squash to rice. Simmer, stirring every 3 to 4 minutes, until liquid is absorbed and bottom of pan is almost dry, about 12 minutes.
Stir in about 1/2 cup hot broth and cook, stirring constantly, until absorbed, about 3 minutes; repeat with additional broth 2 or 3 more times, until rice is al dente. Turn off the heat, stir in remaining 1 tablespoon butter/margarine, Parmesan, sage, and nutmeg; drain excess liquid from spinach and gently fold in spinach and remaining cooked squash. If desired, add up to 1/4 cup additional hot broth to loosen texture of risotto. Top individual servings of risotto with toasted pine nuts; serve immediately.