Preheat the oven to 375 degrees F and line a sheet pan with foil. Spread squash and onion on the pan, toss with oil, sprinkle with salt, and cook for 25-30 minutes or until very soft. Let cool slightly.
Transfer the squash and onions to a blender and puree, adding the broth to make a creamy yet thick consistency.
Warm the rice. Stir squash mixture into rice, season to taste with additional salt if needed, and top with Parmesan cheese (if using) to serve.
Notes
Roast the vegetables until they are soft when poked with a fork to ensure they blend smoothly.
Season to taste with salt to ensure it tastes good to you. (The broth you use will impact saltiness, so it's important to taste and adjust.)
Use fully cooked and soft rice so it has the texture of classic risotto.