Warm liquids. On a back burner, heat 31⁄2 cups water and chicken broth in large saucepan. Cover and keep warm over low heat.

Cook squash. Heat 2 tablespoons olive oil in heavy large nonstick skillet over high heat. Add squash; sprinkle with sea salt and sauté until beginning to brown, stirring often, about 5 minutes. Reduce heat to medium, add chopped sage, and cook until just tender, stirring often, about 8 minutes. Sprinkle Sherry wine vinegar over; toss to incorporate. Transfer squash to plate, and return skillet to burner.

Prepare risotto. Heat remaining 2 tablespoons olive oil in same skillet over medium heat. Add onions and sauté until soft and slightly golden, about 6 minutes. Add arborio rice to pan, stirring until rice begins to turn translucent, about 4 minutes. Add 1 cup warm broth mixture and stir until nearly all liquid is absorbed, about 5 minutes. Continue adding broth mixture 1 cup at a time when the liquid in the pan is nearly all absorbed, stirring frequently. After 15 minutes, add the squash, stirring vigorously so that the squash begins to soften. Continue cooking and stirring the risotto, adding broth mixture about a cup at a time until the rice is tender but still firm to bite and the risotto is creamy. (In my case, it took about 30 minutes total for the rice to reach the perfect consistency. I used all of the liquid and the squash became soft and began to break down.)

Transfer squash risotto to large wide serving bowl. Top with parmesan and pancetta. Serve immediately.