You can cook the squash, carrot, and onion mixture up to a day ahead, and store, tightly covered, in the refrigerator. Drain as directed, and proceed. You can substitute reduced-sodium chicken broth and cream of mushroom soup in this recipe with good results.
This was the best squash casserole I have ever fixed! The squash keeps it shape better than in casseroles where it is boiled, and I think it tastes better.
I didn't have cornflakes on hand, so I used cheese cracker crumbs for the topping.