Heat the oven to 400°F with racks in the lower-middle and lowest positions. On a rimmed baking sheet, toss half of the squash (about 2 cups) with the oil and ¼ teaspoon each salt and pepper. Roast on the lowest rack, stirring once about halfway through, until golden brown, about 20 minutes. Remove from the oven and set aside; leave the oven on.

In a large Dutch oven over medium, melt 2 tablespoons butter. Add the onion and ½ teaspoon salt; cook, stirring often, until translucent, about 5 minutes. Add the rice and cook, stirring constantly, until the grains are translucent at the edges, 1 to 2 minutes. Stir in the remaining uncooked squash and the wine; cook, stirring, until the pot is almost dry, about 2 minutes.

Stir in the broth, rosemary, 1 cup water and ½ teaspoon pepper. Cover and place in the oven on the lower-middle rack. Cook until the rice is tender but has not yet absorbed all the liquid and a skewer inserted into the squash meets no resistance, about 45 minutes.

Remove from the oven, uncover and stir in the roasted squash. Add half of the cheese and the remaining 1 tablespoon butter, then stir vigorously until the butter is melted and the risotto is creamy. Let stand uncovered for 5 minutes; the risotto will thicken as it stands.

Taste and season with salt and pepper. Serve with the remaining cheese and additional oil for drizzling.