Step 1. Heat 2 tablespoons oil in a large skillet over medium heat and add the squash. Cook until tender, 5–7 minutes. Add sea salt, pepper, and sage. Set aside until ready to finish the risotto. Bring 5 cups of broth to boil in a saucepan; reduce to a simmer. Save remaining 2 cups.
Step 2. Heat 1 tablespoon each butter and olive oil in a heavy 4–5 quart sauté pan over medium heat. Add onion; sauté for about 2 minutes, or until translucent. Add rice to mixture and stir until all grains are coated.
Step 3. Using a ladle, start adding simmering broth 1/2 cup at a time, stirring continuously. Allow each addition to be fully absorbed before adding more. This will take about 20–25 minutes. The rice will be slightly al dente. Season with salt and pepper to taste, and place away from direct heat until ready to finish.
Step 4. About 5 minutes before serving, bring remaining 2 cups broth to a simmer. Warm squash on the stove. Heat rice mixture over medium heat. Immediately start adding simmering broth, 1/2 cup at a time, stirring until soft and fairly loose. Most of the broth should have evaporated, but it shouldn't be dry (you may not need all 2 cups). The rice should be soft, not mushy. If too dry, add a bit more hot broth. Stir in squash; taste for seasoning. Serve in shallow bowls. Sprinkle Parmigiano-Reggiano on top. Garnish with more sage, if desired, and serve immediately.